cus·tom / ˈkəstəm/ • n. 1. a traditional and widely accepted way of behaving or doing something that is specific to a particular society, place, or time. ∎  [in sing.] a thing that one does habitually: it was my custom to nap for an hour every day. ∎  Law established practice or usage having the force of law or right.2. chiefly Brit. regular dealings with a shop or business by customers.• adj. made or done to order for a particular customer: a custom guitar.

custom habitual practice XII; established usage; tribute, impost XIV; business, patronage XVI. — OF. custome, co(u)stume (mod. coutume) :- *costumne, for *costudne :- L. consuetūdō, -din-, f. consuēscere accustom, accustom oneself, f. CON- + suēscere become accustomed, f. suī g. sg. of refl. pron. ‘oneself’.
So customary liable to customs or dues, holding by custom XVI; accustomed XVII. — medL. custumārius, f. custuma, f. AN. custume. customer †customary tenant; †collector of customs XIV; customary purchaser XV; (colloq.) person (to have to do with) XVI. orig. — AN. custumer, medL. custumārius; in some senses newly f. custom; see -ER 1.

Custom (Jewish): see MINHAG.