What characteristic defines a unilateral contract?

A unilateral contract is defined by its structure where one party makes a promise in exchange for the performance of an act from the other party. Unlike bilateral contracts, which involve mutual promises from both parties, a unilateral contract is only binding on the party making the promise once the other party completes the act. This type of contract is often seen in situations such as reward offers, where one party promises payment to anyone who performs a specific task, like finding a lost pet or completing a challenge.

The characteristic that it must always be in writing is not accurate for unilateral contracts, as they can also be verbal. Similarly, requiring both parties to sign a contract applies mainly to bilateral contracts, where mutual consent and promises are integral. Thus, the key distinguishing feature of a unilateral contract is the promise from one party that creates an obligation only upon the completion of the requested act by the other.