req·ui·site 
(rĕkwĭ-zĭt)
(rĕkwĭ-zĭt)adj.
Required; essential. See Synonyms at indispensable.
n.
Something that is indispensable; a requirement: Algebra is a requisite for taking calculus.
[Middle English, from Latin requīsītus, past participle of requīrere, to require; see REQUIRE.]
requi·site·ly adv.
requi·site·ness n.