unilateral contract and bilateral contract

unilateral contract
n. an agreement to pay in exchange for performance, if the potential performer chooses to act. A "unilateral" contract is distinguished from a "bilateral" contract, which is an exchange of one promise for another. Example of a unilateral contract: "I will pay you $1,000 if you bring my car from Cleveland to San Francisco." Bringing the car is acceptance. The difference is normally only of academic interest.

bilateral contract
n. an agreement in which the parties exchange promises for each to do something in the future. "Susette Seller promises to sell her house to Bobby Buyer and Buyer promises to pay Seller $100,000 for it." This is distinct from a "unilateral contract," in which there is a promise to pay if the other party chooses to do something. "I'll pay you $1,000 if you'll stop smoking." These are basically academic differences which are only important in the rare instance in which one person has acted in anticipation that the other will have obligations as well.