What is Mistake?

Legal Definition
In contract law, a mistake is an erroneous belief, at contracting, that certain facts are true. It can be argued as a defense, and if raised successfully can lead to the agreement in question being found void ab initio or voidable, or alternatively an equitable remedy may be provided by the courts. Common law has identified three different types of mistake in contract: the 'unilateral mistake', the 'mutual mistake' and the 'common mistake'. The distinction between the 'common mistake' and the 'mutual mistake' is important.

Another breakdown in contract law divides mistakes into four traditional categories: unilateral mistake, mutual mistake, mistranscription, and misunderstanding.
-- Wikipedia
Legal Definition
In general, any error or misconception.

In contract law, a situation where the parties did not mean the same thing when they agreed to a term or provision. Also, when at least one contracting party held a belief that was factually or legally false. As a result, the contract may be subject to modification or rescission.

In criminal law, a defense that claims either a misconception of the law or some fact. The defense of mistaking the law almost never succeeds. Mistakes of fact can work as a defense more often, especially if they are reasonable.
Legal Definition
Contracts. An error committed in relation to some matter of fact affecting the rights of one of the parties to a contract.

2. Mistakes in making a contract are distinguished ordinarily into, first, mistakes as to the motive; secondly, mistakes as to the person, with whom the contract is made; thirdly, as to the subject matter of the contract; and, lastly, mistakes of fact and of law. See Story, Eq. Jur. 110; Bouv. Inst. Index, h. t.; Ignorance; Motive.

3. In general, courts of equity will correct and rectify all mistakes in deeds and contracts founded on good consideration. 1 Ves. 317; 2 Atk. 203; Mitf. Pl. 116; 4 Vin. Ab. 277; 13 Vin. Ab. 41; 18 E. Com. Law Reps. 14; 8 Com. Digest, 75; Madd. Ch. Prac. Index, h. t.; 1 Story on Eq. ch. 5, p. 121; Jeremy's Eq. Jurisd. B. 3, part 2, p. 358. See article Surprise.

4. As to mistakes in the names of legatees, see 1 Rop. Leg. 131; Domat, l. 4, t. 2, s. 1, n. 22. As to mistakes made in practice, and as to the propriety or impropriety of taking advantage of them, see Chitt. Pr. Index, h. t. As to mistakes of law in relation to contracts, see 23 Am. Jur. 146 to 166.
-- Bouviers Law Dictionary
Legal Definition
Some unintentional act, omission or error arising from ignorance, surprise, imposition or misplaced confidence. Code Ga. § 3117; 1 Story, Eq. Jur. § 110. That result of ignorance of law or fact which has misled a person to commit that which, if he had not been in error, he would not have done. Jeremy, Eq. Jur. 358. A mistake exists when a person, under some erroneous conviction of law or fact, does, or omits to do, some act which, but for the erroneous conviction, he would not have done or omitted. It may arise either from unconsciousness, ignorance, forgetfulness, imposition, or misplaced confidence. Bisph. Eq. § 185. And see Allen v. Elder, 76 Ga. 677, 2 Am. St Rep. 63; Russell v. Colyar, 4 Heisk. (Tenn.) 154; Peasley v. McFadden, 68 Cal. 611, 10 Pan. 179; Cummins v. Bulgin, 37 N. J. Eq. 476; Chicago, etc., R. Co., v. Hay, 119 III. 493, 10 N. E. 29; McLoney v. Edgar, 7 Pa. Co. Ct. R. 29. Mistake of fact is a mistake not caused by the neglect of a legal duty on the part of the person making the mistake, and consisting in

(1) an unconscious ignorance or forgetfulness of a fact,-past or present, material to the contract; or

(2) belief in the present existence of a thing material to the contract which does not exist, or in the past existence of such a thing which bas not existed. Civ. Code Cal. § 1577. A mistake of law happens when a party, having full knowledge of the facts, comes to an erroneous conclusion as to their legal effect. It is a mistaken opinion or inference, arising from an imperfect or incorrect exercise of the judgment, upon facis as they really are ; and, like a correct opinion, which is law, necessarily presupposes that the person forming it is in full possession of them. The facte precede the law, and the true and false opinion alike imply an acquaintance with them. Neither can exist without it. The one is the result of a correct application to them of legal principles, which every man is presumed to know, and is called "law ;" the other, the result of a faulty application, and is called a "mistake of law." Hurd v. Hall, 12 Wis. 124. Mutual mistake is where the parties have a common intention, but it is induced by a common or mutual mistake.
-- Black's Law Dictionary
Legal Definition
An intentional act or omission from ignorance, surprise, imposition or misplaced confidence. See 76 Ga. 674, 2 Am. St. Rep. 63.
-- Ballentine's Law Dictionary