ubiquitous – Being present everywhere.
ulterior – Not so pertinent as something else to the matter spoken of.
ultimate – Beyond which there is nothing else.
ultimatum – A final statement or proposal, as concerning terms or conditions.
ultramundane – Pertaining to supernatural things or to another life.
ultramontane – Beyond the mountains, especially beyond the Alps (that is, on their Italian side).
umbrage – A sense of injury.
unaccountable – Inexplicable.
unaffected – Sincere.
unanimous – Sharing the same views or sentiments.
unanimity – The state or quality of being of one mind.
unavoidable – Inevitable.
unbearable – Unendurable.
unbecoming – Unsuited to the wearer, place, or surroundings.
unbelief – Doubt.
unbiased – Impartial, as judgment.
unbridled – Being without restraint.
uncommon – Rare.
unconscionable – Ridiculously or unjustly excessive.
unconscious – Not cognizant of objects, actions, etc.
unction – The art of anointing as with oil.
unctuous – Oily.
undeceive – To free from deception, as by apprising of the real state of affairs.
undercharge – To make an inadequate charge for.
underexposed – Insufficiently exposed for proper or full development, as negatives in photography.
undergarment – A garment to be worn under the ordinary outer garments.
underman – To equip with less than the full complement of men.
undersell – To sell at a lower price than.
undersized – Of less than the customary size.
underhanded – Clandestinely carried on.
underlie – To be the ground or support of.
underling – A subordinate.
undermine – To subvert in an underhand way.
underrate – To undervalue.
understate – To fail to put strongly enough, as a case.
undervalue – To underestimate.
underworld – Hades.
underwrite – To issue or be party to the issue of a policy of insurance.
undue – More than sufficient.
undulate – To move like a wave or in waves.
undulous – Resembling waves.
unfavorable – Adverse.
ungainly – Clumsy.
unguent – Any ointment or lubricant for local application.
unicellular – Consisting of a single cell.
univalence – Monovalency.
unify – To cause to be one.
unique – Being the only one of its kind.
unison – A condition of perfect agreement and accord.
unisonant – Being in a condition of perfect agreement and accord.
Unitarian – Pertaining to a religious body that rejects the doctrine of the Trinity.
unlawful – Illegal.
unlimited – Unconstrained.
unnatural – Artificial.
unnecessary – Not essential under the circumstances.
unsettle – To put into confusion.
unsophisticated – Showing inexperience.
unspeakable – Abominable.
untimely – Unseasonable.
untoward – Causing annoyance or hindrance.
unutterable – Inexpressible.
unwieldy – Moved or managed with difficulty, as from great size or awkward shape.
unwise – Foolish.
unyoke – To separate.
up-keep – Maintenance.
upbraid – To reproach as deserving blame.
upcast – A throwing upward.
upheaval – Overthrow or violent disturbance of established order or condition.
upheave – To raise or lift with effort.
uppermost – First in order of precedence.
uproarious – Noisy.
uproot – To eradicate.
upturn – To throw into confusion.
urban – Of, or pertaining to, or like a city.
urbanity – Refined or elegant courtesy.
urchin – A roguish, mischievous boy.
urgency – The pressure of necessity
usage – Treatment.
usurious – Taking unlawful or exorbitant interest on money loaned.
usurp – To take possession of by force.
usury – The demanding for the use of money as a loan, a rate of interest beyond what is allowed by law.
utilitarianism – The ethical doctrine that actions are right because they are useful or of beneficial tendency.
utility – Fitness for some desirable practical purpose.
utmost – The greatest possible extent.