custom

/ˈkʌs·təm/ n [C/U] (TRADITION)

Definition

› a way of behaving or a belief that has been established for a long time among a group of people: [C] It was a time-honored custom of the club that blue chairs were reserved for senior members only. [C] She’s studying the language and customs of the Hopi Indians.

(Definition of custom n (TRADITION) from the Cambridge Academic Content Dictionary © Cambridge University Press)