One of three requisites to a valid contract under common law (the other two being an offer and consideration).
A contract is a legally binding agreement between two or more parties which starts with an offer from one person but which does not become a contract until the other party signifies an unequivocal willingness to accept the terms of that offer.
The moment of acceptance is the moment from which a contract is said to exist, and not before.
Acceptance need not always be direct and can, in certain circumstances, be implied by conduct (most notably by handshake, pictured).