Merriam-Webster Collegiate Dictionary - imperative

 I. adjective  Etymology: Middle English imperatyf, from Late Latin imperativus, from Latin imperatus, past participle of imperare to command — more at emperor  Date: 15th century  1.  a. of, relating to, or constituting the grammatical mood that expresses the will to influence the behavior of another  b. expressive of a command, entreaty, or exhortation  c. having power to restrain, control, and direct  2. not to be avoided or evaded ; necessary an ~ duty  Synonyms: see masterful  • ~ly adverb  • ~ness noun  II. noun  Date: 1530  1. the ~ mood or a verb form or verbal phrase expressing it  2. something that is ~: as  a. command, order  b. rule, guide  c. an obligatory act or duty  d. an ~ judgment or proposition