Unilateral contracts are one sided. In a unilateral contract, a promise on one side is exchanged for an act or forbearance on the other side. The offeror, makes a promise in exchange for an act by the offeree. If the offeree acts on the offeror's promise, the offeror is legally obligated to fulfill the contract. However, the offeree cannot be forced to act because no return promise has been made to the offeror. Only on performance by the offeree, the offeror's enforceable promise exists.
The following is an example of a case law defining unilateral contract:
A contract is unilateral when one party furnishes no consideration to the other, and does not obligate himself to do anything that may result in injury to himself or benefit to the other. [Petroleum Research Corp. v. Barnsdall Refining Corp., 188 Okla. 62 (Okla. 1940)]