A thing that must hold for the whole to be acceptable. A requirement removes a choice on purpose.
Built on require, from Latin requirere (to seek again, to need): re- (again) + quaerere (to seek); a requirement is what the insistent seeking of the root settles on as necessary for the whole to hold.
A thing that must hold for the whole to be acceptable. Like require, its root's seeking has hardened into something no longer negotiable — a requirement removes a choice on purpose.