Consideration

Consideration is the value exchanged in a contract. Each side has to give up something (money, a service, a promise, a right) for the agreement to be enforceable. A promise to do something for nothing in return is generally not a contract, it is a gift, and courts will not enforce it the same way.

Consideration does not have to be money, and it does not have to be equal on both sides. It just has to be real. This is why some contracts recite a nominal amount ("for one dollar and other good and valuable consideration") to make the exchange explicit.