Unilateral contract

Definition
In a unilateral contract, one party, (the offeror) makes a promise in exchange for an act (or abstention from acting) by another party (the offeree). If the offeree acts on the offeror's promise, the offeror is legally obligated to fulfill the contract, but the offeree is not required to act (or not to act), because no return promise has been made to the offeror. After the offeree has performed, the offeror must fulfill his/her promise.