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.
In Book 9(1) of Halsbury's Laws of England, 4th Edition (2007), the authors, using no less than 13 footnotes throughout the short excerpt, describe acceptance as follows:
"An acceptance of an offer is an indication, express or implied, by the offeree made whilst the offer remains open, and in the manner requested in that offer of the offeree's willingness to be bound unconditionally to a contract with the offeror on the terms stated in the offer."
In Greba, Justice Matheson of the Saskatchewan Court of Queen's Bench adopted these words:
"... acceptance ... a final and unqualified expression of assent to the terms of an offer."
"Acceptance means the signification by the offeree of his willingness to enter into a contract with the offeror on the terms offered to him by the latter. Without an acceptance there can be no contract ...."
"In the ordinary case ... there is a manifestation of willingness to be bound by one party followed by a manifestation of an assent by the other by words or conduct, generally and usefully called offer and acceptance."
"Each of (these) definitions contemplates a willingness, either in words or by conduct, to be bound by the terms of the offer. But each definition also entails a manifestation, or a signification, or an indication of such willingness."
Acceptance need not always be direct and can, in certain circumstances, be implied by conduct (most notably by handshake, pictured).
Further, the exact moment of acceptance may not always be readily discernible. That, for want of more, would not suffice in avoiding the terms of a contract where actions or other evidence of acceptance is present.
In Krakana, Justice Thompson wrote:
"The whole transaction is to be looked at and an inquiry into the precise time at which the different parts of one transaction take place is unnecessary if in fact it can be determined from the whole that a single contract was intended and resulted."
At §2-204(2) of the United States Commercial Code:
"An agreement sufficient to constitute a contract for sale may be found even if the moment of its making is undetermined."