What is Mutual Assent?

Legal Definition
Agreement by both parties to a contract. Mutual assent must be proven objectively, and is often established by showing an offer and acceptance (e.g., an offer to do X in exchange for Y, followed by an acceptance of that offer).
Legal Definition
The meeting of the minds of both or all the parties to a contract; the fact that each agrees to all the terms and conditions, in the same sense and with the same meaning as the others. Insurance Co., v. Young, 23 Wall. 107, 23 In Ed. 152.
-- Black's Law Dictionary