What type of contract involves a promise from only one party?

A unilateral contract is characterized by a promise made by only one party, which can be contingent on the other party's performance of a certain action. In this type of agreement, one party offers something of value in exchange for the performance of a specific act by another party. The classic example is a reward contract, such as someone offering a reward for finding a lost pet—only the person making the offer is bound to fulfill the promise of the reward once the act of finding the pet is completed.

In contrast, a bilateral contract involves a mutual agreement where both parties make promises to each other, creating reciprocal obligations. An implied contract is formed by conduct rather than explicit written or spoken words. An express contract, meanwhile, is explicitly articulated either in writing or verbally, but this type of contract also typically involves promises from both parties. Therefore, the defining feature of a unilateral contract is the one-sided promise, making it distinct from these other types of contracts.