ichthyic – Fish-like.
ichthyology – The branch of zoology that treats of fishes.
ichthyosaurs – A fossil reptile.
icily – Frigidly.
iciness – The state of being icy.
icon – An image or likeness.
iconoclast – An image-breaker.
idealize – To make to conform to some mental or imaginary standard.
idiom – A use of words peculiar to a particular language.
idiosyncrasy – A mental quality or habit peculiar to an individual.
idolize – To regard with inordinate love or admiration.
ignoble – Low in character or purpose.
ignominious – Shameful.
Iliad – A Greek epic poem describing scenes from the siege of Troy.
illegal – Not according to law.
illegible – Undecipherable.
illegitimate – Unlawfully begotten.
illiberal – Stingy.
illicit – Unlawful.
illimitable – Boundless.
illiterate – Having little or no book-learning.
ill–natured – Surly.
illogical – Contrary to the rules of sound thought.
illuminant – That which may be used to produce light.
illuminate – To supply with light.
illumine – To make bright or clear.
illusion – An unreal image presented to the senses.
illusive – Deceptive.
illusory – Deceiving or tending to deceive, as by false appearance.
imaginable – That can be imagined or conceived in the mind.
imaginary – Fancied.
imbibe – To drink or take in.
imbroglio – A misunderstanding attended by ill feeling, perplexity, or strife.
imbrue – To wet or moisten.
imitation – That which is made as a likeness or copy.
imitator – One who makes in imitation.
immaculate – Without spot or blemish.
immaterial – Of no essential consequence.
immature – Not full-grown.
immeasurable – Indefinitely extensive.
immense – Very great in degree, extent, size, or quantity.
immerse – To plunge or dip entirely under water or other fluid.
immersion – The act of plunging or dipping entirely under water or another fluid.
immigrant – A foreigner who enters a country to settle there.
immigrate – To come into a country or region from a former habitat.
imminence – Impending evil or danger.
imminent – Dangerous and close at hand.
immiscible – Separating, as oil and water.
immoral – Habitually engaged in licentious or lewd practices.
immortalize – To cause to last or to be known or remembered throughout a great or indefinite length of time.
immovable – Steadfast.
immune – Exempt, as from disease.
immutable – Unchangeable.
impair – To cause to become less or worse.
impalpable – Imperceptible to the touch.
impartial – Unbiased.
impassable – That can not be passed through or over.
impassible – Not moved or affected by feeling.
impassive – Unmoved by or not exhibiting feeling.
impatience – Unwillingness to brook delays or wait the natural course of things.
impeccable – Blameless.
impecunious – Having no money.
impede – To be an obstacle or to place obstacles in the way of.
impel – To drive or urge forward.
impend – To be imminent.
imperative – Obligatory.
imperceptible – Indiscernible.
imperfectible – That can not be perfected.
imperil – To endanger.
imperious – Insisting on obedience.
impermissible – Not permissible.
impersonal – Not relating to a particular person or thing.
impersonate – To appear or act in the character of.
impersuadable – Unyielding.
impertinence – Rudeness.
imperturbable – Calm.
impervious – Impenetrable.
impetuosity – Rashness.
impetuous – Impulsive.
impetus – Any impulse or incentive.
impiety – Irreverence toward God.
impious – Characterized by irreverence or irreligion.
implausible – Not plausible.
impliable – Capable of being inferred.
implicate – To show or prove to be involved in or concerned
implicit – Implied.
imply – To signify.
impolitic – Inexpedient.
importation – The act or practice of bringing from one country into another.
importunate – Urgent in character, request, or demand.
importune – To harass with persistent demands or entreaties.
impotent – Destitute of or lacking in power, physical, moral, or intellectual.
impoverish – To make indigent or poor.
impracticable – Not feasible.
impregnable – That can not be taken by assault.
impregnate – To make pregnant.
impromptu – Anything done or said on the impulse of the moment.
improper – Not appropriate, suitable, or becoming.
impropriety – The state or quality of being unfit, unseemly, or inappropriate.
improvident – Lacking foresight or thrift.
improvise – To do anything extemporaneously or offhand.
imprudent – Heedless.
impudence – Insolent disrespect.
impugn – To assail with arguments, insinuations, or accusations.
impulsion – Impetus.
impulsive – Unpremeditated.
impunity – Freedom from punishment.
impure – Tainted.
impute – To attribute.
inaccessible – Difficult of approach.
inaccurate – Not exactly according to the facts.
inactive – Inert.
inadequate – Insufficient.
inadmissible – Not to be approved, considered, or allowed, as testimony.
inadvertent – Accidental.
inadvisable – Unadvisable.
inane – Silly.
inanimate – Destitute of animal life.
inapprehensible – Not to be understood.
inapt – Awkward or slow.
inarticulate – Speechless.
inaudible – That can not be heard.
inborn – Implanted by nature.
inbred – Innate.
incandescence – The state of being white or glowing with heat.
incandescent – White or glowing with heat.
incapacitate – To deprive of power, capacity, competency, or qualification.
incapacity – Want of power to apprehend, understand, and manage.
incarcerate – To imprison.
incendiary – Chemical or person who starts a fire-literally or figuratively.
incentive – That which moves the mind or inflames the passions.
inception – The beginning.
inceptive – Beginning.
incessant – Unceasing.
inchmeal – Piecemeal.
inchoate – Incipient.
inchoative – That which begins, or expresses beginning.
incidence – Casual occurrence.
incident – A happening in general, especially one of little importance.
incidentally – Without intention.
incinerate – To reduce to ashes.
incipience – Beginning.
incipient – Initial.
incisor – A front or cutting tooth.
incite – To rouse to a particular action.
incitement – That which moves to action, or serves as an incentive or stimulus.
incoercible – Incapable of being forced, constrained, or compelled.
incoherence – Want of connection, or agreement, as of parts or ideas in thought, speech, etc.
incoherent – Not logically coordinated, as to parts, elements, or details.
incombustible – That can not be burned.
incomparable – Matchless.
incompatible – Discordant.
incompetence – General lack of capacity or fitness.
incompetent – Not having the abilities desired or necessary for any purpose.
incomplete – Lacking some element, part, or adjunct necessary or required.
incomprehensible – Not understandable.
incompressible – Resisting all attempts to reduce volume by pressure.
inconceivable – Incomprehensible.
incongruous – Unsuitable for the time, place, or occasion.
inconsequential – Valueless.
inconsiderable – Small in quantity or importance.
inconsistent – Contradictory.
inconstant – Changeable.
incontrovertible – Indisputable.
inconvenient – Interfering with comfort or progress.
indefensible – Untenable.
indefinitely – In a vague or uncertain way.
indelible – That can not be blotted out, effaced, destroyed, or removed.
indescribable – That can not be described.
indestructible – That can not be destroyed.
indicant – That which points out.
indicator – One who or that which points out.
indict – To find and declare chargeable with crime.
indigence – Poverty.
indigenous – Native.
indigent – Poor.
indigestible – Not digestible, or difficult to digest.
indigestion – Difficulty or failure in the alimentary canal in changing food into absorptive nutriment.
indignant – Having such anger and scorn as is aroused by meanness or wickedness.
indignity – Unmerited contemptuous conduct or treatment.
indiscernible – Not perceptible.
indiscreet – Lacking wise judgment.
indiscriminate – Promiscuous.
indispensable – Necessary or requisite for the purpose.
indistinct – Vague.
indivertible – That can not be turned aside.
indivisible – Not separable into parts.
indolence – Laziness.
indolent – Habitually inactive or idle.
indomitable – Unconquerable.
induct – To bring in.
indulgence – The yielding to inclination, passion, desire, or propensity in oneself or another.
indulgent – Yielding to the desires or humor of oneself or those under one’s care.
inebriate – To intoxicate.
inedible – Not good for food.
ineffable – Unutterable.
inefficient – Not accomplishing an intended purpose.
inefficiency – That which does not accomplish an intended purpose.
ineligible – Not suitable to be selected or chosen.
inept – Not fit or suitable.
inert – Inanimate.
inestimable – Above price.
inevitable – Unavoidable.
inexcusable – Not to be justified.
inexhaustible – So large or furnishing so great a supply as not to be emptied, wasted, or spent.
inexorable – Unrelenting.
inexpedient – Unadvisable.
inexpensive – Low-priced.
inexperience – Lack of or deficiency in experience.
inexplicable – Such as can not be made plain.
inexpressible – Unutterable.
inextensible – Of unchangeable length or area.
infallible – Exempt from error of judgment, as in opinion or statement.
infamous – Publicly branded or notorious, as for vice, or crime.
infamy – Total loss or destitution of honor or reputation.
inference – The derivation of a judgment from any given material of knowledge on the ground of law.
infernal – Akin to or befitting hell or its occupants.
infest – To be present in such numbers as to be a source of annoyance, trouble, or danger.
infidel – One who denies the existence of God.
infidelity – Disloyalty.
infinite – Measureless.
infinity – Boundless or immeasurable extension or duration.
infirm – Lacking in bodily or mental strength.
infirmary – A place for the reception or treatment of the sick.
infirmity – A physical, mental, or moral weakness or flaw.
inflammable – Easily set on fire or excited.
inflammation – A morbid process in some part of the body characterized by heat, swelling, and pain.
inflexible – That can not be altered or varied.
influence – Ability to sway the will of another.
influential – Having the power to sway the will of another.
influx – Infusion.
infrequence – Rareness.
infrequent – Uncommon.
infringe – To trespass upon.
infuse – To instill, introduce, or inculcate, as principles or qualities.
infusion – The act of imbuing, or pouring in.
ingenious – Evincing skill, originality, or cleverness, as in contrivance or arrangement.
ingenuity – Cleverness in contriving, combining, or originating.
ingenuous – Candid, frank, or open in character or quality.
inglorious – Shameful.
ingraft – To set or implant deeply and firmly.
ingratiate – To win confidence or good graces for oneself.
ingratitude – Insensibility to kindness.
ingredient – Component.
inherence – The state of being permanently existing in something.
inherent – Intrinsic.
inhibit – To hold back or in.
inhospitable – Not disposed to entertain strangers gratuitously.
inhuman – Savage.
inhume – To place in the earth, as a dead body.
inimical – Adverse.
iniquity – Gross wrong or injustice.
initiate – To perform the first act or rite.
inject – To introduce, as a fluid, by injection.
injunction – Mandate.
inkling – A hint.
inland – Remote from the sea.
inlet – A small body of water leading into a larger.
inmost – Deepest within.
innocuous – Harmless.
innovate – To introduce or strive to introduce new things.
innuendo – Insinuation.
innumerable – Countless.
inoffensive – Causing nothing displeasing or disturbing.
inopportune – Unsuitable or inconvenient, especially as to time
inquire – To ask information about.
inquisition – A court or tribunal for examination and punishment of heretics.
inquisitive – Given to questioning, especially out of curiosity.
inquisitor – One who makes an investigation.
inroad – Forcible encroachment or trespass.
insatiable – That desires or craves immoderately or unappeasably.
inscribe – To enter in a book, or on a list, roll, or document, by writing.
inscrutable – Impenetrably mysterious or profound.
insecure – Not assured of safety.
insensible – Imperceptible.
insentient – Lacking the power of feeling or perceiving.
inseparable – That can not be separated.
insidious – Working ill by slow and stealthy means.
insight – Intellectual discernment.
insignificance – Lack of import or of importance.
insignificant – Without importance, force, or influence.
insinuate – To imply.
insipid – Tasteless.
insistence – Urgency.
insistent – Urgent.
insolence – Pride or haughtiness exhibited in contemptuous and overbearing treatment of others.
insolent – Impudent.
insomnia – Sleeplessness.
inspector – An official appointed to examine or oversee any matter of public interest or importance.
instance – A single occurrence or happening of a given kind.
instant – A very brief portion of time.
instantaneous – Done without perceptible lapse of time.
instigate – To provoke.
instigator – One who incites to evil.
instill – To infuse.
instructive – Conveying knowledge.
insufficiency – Inadequacy.
insufficient – Inadequate for some need, purpose, or use.
insular – Pertaining to an island.
insulate – To place in a detached state or situation.
insuperable – Invincible.
insuppressible – Incapable of being concealed.
insurgence – Uprising.
insurgent – One who takes part in forcible opposition to the constituted authorities of a place.
insurrection – The state of being in active resistance to authority.
intangible – Not perceptible to the touch.
integrity – Uprightness of character and soundness of moral principle.
intellect – The faculty of perception or thought.
intellectual – Characterized by intelligence.
intelligence – Capacity to know or understand.
intelligible – Comprehensible.
intemperance – Immoderate action or indulgence, as of the appetites.
intension – The act of stringing or stretching, or state of being strained.
intensive – Adding emphasis or force.
intention – That upon which the mind is set.
interact – To act reciprocally.
intercede – To mediate between persons.
intercept – To interrupt the course of.
intercession – Entreaty in behalf of others.
intercessor – A mediator.
interdict – Authoritative act of prohibition.
interim – Time between acts or periods.
interlocutor – One who takes part in a conversation or oral discussion.
interlude – An action or event considered as coming between others of greater length.
intermediate – Being in a middle place or degree or between extremes.
interminable – Having no limit or end.
intermission – A recess.
intermit – To cause to cease temporarily.
intermittent – A temporary discontinuance.
interpolation – Verbal interference.
interpose – To come between other things or persons.
interposition – A coming between.
interpreter – A person who makes intelligible the speech of a foreigner by oral translation.
interrogate – To examine formally by questioning.
interrogative – Having the nature or form of a question.
interrogatory – A question or inquiry.
interrupt – To stop while in progress.
intersect – To cut through or into so as to divide.
intervale – A low tract of land between hills, especially along a river.
intervene – To interfere for some end.
intestacy – The condition resulting from one’s dying not having made a valid will.
intestate – Not having made a valid will.
intestine – That part of the digestive tube below or behind the stomach, extending to the anus.
intimacy – Close or confidential friendship.
intimidate – To cause to become frightened.
intolerable – Insufferable.
intolerance – Inability or unwillingness to bear or endure.
intolerant – Bigoted.
intoxicant – Anything that unduly exhilarates or excites.
intoxicate – To make drunk.
intracellular – Occurring or situated within a cell.
intramural – Situated within the walls of a city.
intrepid – Fearless and bold.
intricacy – Perplexity.
intricate – Difficult to follow or understand.
intrigue – A plot or scheme, usually complicated and intended to accomplish something by secret ways.
intrinsic – Inherent.
introductory – Preliminary.
introgression – Entrance.
intromit – To insert.
introspect – To look into.
introspection – The act of observing and analyzing one’s own thoughts and feelings.
introversion – The act of turning or directing inward, physically or mentally.
introvert – To turn within.
intrude – To come in without leave or license.
intrusion – The act of entering without warrant or invitation; encroachment.
intuition – Instinctive knowledge or feeling.
inundate – To fill with an overflowing abundance.
inundation – Flood.
inure – To harden or toughen by use, exercise, or exposure.
invalid – Having no force, weight, or cogency.
invalid – One who is disabled by illness or injury.
invalidate – To render of no force or effect.
invaluable – Exceedingly precious.
invariable – Unchangeable.
invasion – Encroachment, as by an act of intrusion or trespass.
invective – An utterance intended to cast censure, or reproach.
inveigh – To utter vehement censure or invective.
inventive – Quick at contrivance.
inverse – Contrary in tendency or direction.
inversion – Change of order so that the first shall become last and the last first.
invert – To turn inside out, upside down, or in opposite direction.
investigator – One who investigates.
investor – One who invests money.
inveterate – Habitual.
invidious – Showing or feeling envy.
invigorate – To animate.
invincible – Not to be conquered, subdued, or overcome.
inviolable – Incapable of being injured or disturbed.
invoke – To call on for assistance or protection.
involuntary – Unwilling.
involution – Complication.
involve – To draw into entanglement, literally or figuratively.
invulnerable – That can not be wounded or hurt.
inwardly – With no outward manifestation.
iota – A small or insignificant mark or part.
irascible – Prone to anger.
irate – Moved to anger.
ire – Wrath.
iridescence – A many-colored appearance.
iridescent – Exhibiting changing rainbow-colors due to the interference of the light.
irk – To afflict with pain, vexation, or fatigue.
irksome – Wearisome.
irony – Censure or ridicule under cover of praise or compliment.
irradiance – Luster.
irradiate – To render clear and intelligible.
irrational – Not possessed of reasoning powers or understanding.
irreducible – That can not be lessened.
irrefragable – That can not be refuted or disproved.
irrefrangible – That can not be broken or violated.
irrelevant – Inapplicable.
irreligious – Indifferent or opposed to religion.
irreparable – That can not be rectified or made amends for.
irrepressible – That can not be restrained or kept down.
irresistible – That can not be successfully withstood or opposed.
irresponsible – Careless of or unable to meet responsibilities.
irreverence – The quality showing or expressing a deficiency of veneration, especially for sacred things.
irreverent – Showing or expressing a deficiency of veneration, especially for sacred things.
irreverential – Showing or expressing a deficiency of veneration, especially for sacred things.
irreversible – Irrevocable.
irrigant – Serving to water lands by artificial means.
irrigate – To water, as land, by ditches or other artificial means.
irritable – Showing impatience or ill temper on little provocation.
irritancy – The quality of producing vexation.
irritant – A mechanical, chemical, or pathological agent of inflammation, pain, or tension.
irritate – To excite ill temper or impatience in.
irruption – Sudden invasion.
isle – An island.
islet – A little island.
isobar – A line joining points at which the barometric pressure is the same at a specified moment.
isochronous – Relating to or denoting equal intervals of time.
isolate – To separate from others of its kind.
isothermal – Having or marking equality of temperature.
itinerant – Wandering.
itinerary – A detailed account or diary of a journey.
itinerate – To wander from place to place.