A contract is an act as it were against an act.
Contractus Est Quasi Actus Contra Actum is a latin term which translate into a contract is an act as it were against an act. It aims at explaining that any contract is in the nature of an act against an act. In the formation of a contact there must be an act against a consideration. Consideration is the very basic foundation of ever contract. The law and the maxim states that any promise of any act without consideration void ad initio. There are certain aspects of a valid consideration. The consideration should always be at the desire of the person making the promiseor any other person authorised to do so. The act of consideration may occur in the past, present or future, but its presence in the contract is important. This considered to be a vital element in the law of contracts. In other words it can be described as a benefit which must be bargained for between the parties, and is the essential reason for a party entering into a contract.
Quasi is a latin term which is used in legal phraseology to indicate upon situations where two subjects are compared and it shows that that they resembles each other but at the same time have different characteristics which make them unique. The maxim Contractus Est Quasi Actus Contra Actum is also based upon this concept. It deals with the formation of contract and states that a contract is essentially an act in the nature that as if it were against an act.
For example if a contract lies between A and B for the supply of stationery for 6 months at the rate of Rs. 50000 per month then the amount of Rs. 50000 is considered to be the consideration, against the act of supplying stationeries
In the case of Manby v Scott it is said, that agreement is a word compounded of two words, viz. aggregatio and mentium; that aggreamentum is aggregatio mentium; or thusaggreamentum is no other but a union or conjunction of two minds in any matter or thing done, or to be done; according to that definition of Sir Edward Coke , contractus estquasi actus contra actum but a feme covert cannot give a mutual assent of mind.
In the case of Cotton v Wescott it was then urged, that here is another consideration for another promise in the declaration in consideration that the plaintiff should board the defendant for a quarter of a year to come, he did assume to pay him tantum, quantum meritus esset, and doth alledge, that he did board him for eleven weeks, and so of his own shewing; it appears that he had not boarded him for the whole quarter. Yet this is good, for the words licet sæpius requisitus; and contractus est quasi actus contra actum; and if one contracts to pay unto another for anything, tantum, quantum meruit, as for a quarters board, if he will go away two or three days after, he shall pay for the residue.
Edited by Vigneshwar Ramasubramania
Approved & Published – Sakshi Raje
 Manby v Scott,  1 WLUK 186
 Cotton v Wescott,  1 WLUK 143