What is Habit?

Legal Definition
A habit (or wont) is a routine of behavior that is repeated regularly and tends to occur subconsciously.

In the American Journal of Psychology (1903) it is defined in this way: "A habit, from the standpoint of psychology, is a more or less fixed way of thinking, willing, or feeling acquired through previous repetition of a mental experience." Habitual behavior often goes unnoticed in persons exhibiting it, because a person does not need to engage in self-analysis when undertaking routine tasks. Habits are sometimes compulsory. The process by which new behaviours become automatic is habit formation. Old habits are hard to break and new habits are hard to form because the behavioural patterns we repeat are imprinted in our neural pathways, but it is possible to form new habits through repetition.

As behaviors are repeated in a consistent context, there is an incremental increase in the link between the context and the action. This increases the automaticity of the behavior in that context. Features of an automatic behavior are all or some of: efficiency, lack of awareness, unintentionality, uncontrollability.
-- Wikipedia
Legal Definition
A disposition or condition of the body or mind acquired by custom or a frequent repetition of the same act. See 2 Mart. Lo. Rep. N. S. 622.

2. The habit of dealing has always an important bearing upon the construction of commercial contracts. A ratification will be inferred from the mere habit of dealing between the parties; as, if a broker has been accustomed to settle losses on policies in a particular manner, without any objection being made, or with the silent approbation of his principal, and he should afterward settle other policies in the same manner, to which no objection should be made within a reasonable time, a just presumption would arise of an implied ratification; for if the principal did not agree to such settlement he should have declared his dissent. 2 Bouv. Inst. 1313-14.
-- Bouviers Law Dictionary
Legal Definition
A disposition or condition of the body or mind acquired by custom or a usual repetition of the same act or function. Knickerbocker L. Ins. Co, v. Foley, 105 U. S. 354, 26 L. Ed. 1055; Conner v. Citizens' St. R. Co.., 146 Ind. 430, 45 N. E. 662; State v. Skillicorn, 104 Iowa, 97, 73 N. W. 503; State v. Robinson. Ill Ala. 482, 20 South. 30. See Habit and repute.
-- Black's Law Dictionary
Legal Definition
Customary or ordinary course of conduct.
-- Ballentine's Law Dictionary