A contract in which only one party has to fulfill his obligation at the time of the formation of the contract, the other party having fulfilled his obligation at the time of the contract or before the contract comes into existence is known as

A unilateral contract is one where only one party makes a promise that becomes enforceable when the other party performs an act. At formation, one party is already obligated (has performed or will perform), while the other party's obligation is contingent on performance. Classic example is a reward offer - the offeror is bound to pay when the offeree completes the act. In bilateral contracts, both parties exchange mutual promises simultaneously.