late 14c., "action of altering," from Old French alteracion (14c.) "change, alteration," and directly from Medieval Latin alterationem (nominative alteratio), noun of action from past participle stem of Late Latin alterare (see alter). Meaning "change in character or appearance" is from 1530s; that of "change in ready-made clothes to suit a customer's specifications" is from 1901. Related: Alterations.