Merriam-Webster's Collegiate® Dictionary, 11th Edition
IMPERATIVE
I. adjective see: emperor
Date: 15th century
1. of, relating to, or constituting the grammatical mood that expresses the will to influence the behavior of another, expressive of a command, entreaty, or exhortation, having power to restrain, control, and direct, not to be avoided or evaded ; necessary , see: masterful imperatively adverb imperativeness noun
II. noun
Date: 1530
the imperative mood or a verb form or verb al phrase expressing it, something that is imperative: as, command , order , rule , guide , an obligatory act or duty, an imperative judgment or proposition