A contract in which only one party makes an express promise, or undertakes a performance without first securing a reciprocal agreement from the other party.
In a unilateral, or one-sided, contract, one party, known as the offeror, makes a promise in exchange for an act (or abstention from acting) by another party, known as the offeree. If the offeree acts on the offeror's promise, the offeror is legally obligated to fulfill the contract, but an offeree cannot be forced to act (or not act), because no return promise has been made to the offeror. After an offeree has performed, only one enforceable promise exists, that of the offeror.
A unilateral contract differs from a bilateral contract, in which the parties exchange mutual promises. Bilateral contracts are commonly used in business transactions; a sale of goods is a type of bilateral contract.
Reward offers are usually unilateral contracts. The offeror (the party offering the reward) cannot impel anyone to fulfill the reward offer. An offeree can sue for breach of contract, however, if the offeror does not provide the reward after the offeree has fulfilled the contract's requirements.