noun
custom; habit; the way things are
Often used in fixed expressions like 世の習い (the way of the world) or 習いとなる (to become a habit). Describes a common or expected pattern of behavior or life.
世の習いで、金持ちはますます金持ちになる。
As is the way of the world, the rich get richer.
早起きが彼の習いとなった。
Waking up early became his habit.
習慣 is the common word for a personal habit or custom. 習い is more literary and often appears in set phrases describing general truths or natural tendencies.
慣習 refers to social customs or conventions. 習い is broader and can refer to both personal habits and universal patterns.
Derived from the verb 習う (ならう, 'to learn'), originally meaning something learned and thus habitual. The exact historical development is uncertain, but the meaning shifted from 'learned behavior' to 'custom/habit'.