Each of these words refers to an accustomed or established way of doing things. Each can be applied to the activities of people, animals, or entire communities. Custom refers particularly to the practice and preservation of social activity or usage: "It is a community custom to go to church on Sunday." Practice is closely allied in meaning to custom but applies particularly to an unvarying procedure: "It is the practice of a careful man to balance his checkbook." Habit, applied especially to people and animals, refers to the repetition of an action so constantly that the act becomes natural or spontaneous: "John has a habit of counting to 10 before he answers any question." "Our dog has a habit of turning around before lying down."
Dictionary of problem words and expressions. Harry Shaw. 1975.