Unilateral contract
Contract Law

A unilateral contract is one where a promise is made by only one party to the contract, and the contract is performed when an act or some other consideration is executed by the other party. The promise becomes binding on the offeror when the act is performed.

For instance, a person may offer to give £1 to the first person that finds a solution to a problem. Delivery of the solution to the offeror binds the offeror to pay £1.