Definition
A unilateral contract is a contract created by an offer than can only be accepted by performance.
Overview
In a unilateral contract, there is an express offer that payment is made only by a party's performance. Another example of a unilateral contract is a reward or a contest.
In a unilateral contract, the offeror may revoke the offer before the offeree's performance begins. Typically the revocation needs to be express.
Similar to contract law in general, specific guidelines on unilateral contracts are governed by state laws, rather than federal laws.
Further Reading
For more on unilateral contracts, see this Mississippi Law Journal article, this Washington University Law Review article, and this DePaul Law Review article.