What type of contract involves one party making a promise in exchange for an act?

A unilateral contract is characterized by one party making a promise in exchange for an act performed by another party. This type of contract is distinct in that the offeror commits to fulfill their promise only if the other party takes the specified action, which means the contract is formed through performance rather than mutual promises.

For example, a classic instance of a unilateral contract is a reward offer for finding a lost pet. The offeror promises to pay a reward if someone finds and returns the pet, but the contract is only formed when the act of finding and returning the pet is completed.

This concept contrasts sharply with bilateral contracts, where both parties exchange mutual promises. An implied contract involves unwritten and unspoken agreements based on the actions of the parties, while a void contract is one that is not legally enforceable from the outset. Understanding these distinctions helps clarify why a unilateral contract specifically involves one party's promise in exchange for an act.