Source : Webster's Revised Unabridged Dictionary (1913)

Imperative \Im*per"a*tive\, a. [L. imperativus, fr. imperare to
   command; pref. im- in + parare to make ready, prepare: cf. F.
   imp['e]ratif. See {Perade}, and cf. {Empire}.]
   1. Expressive of command; containing positive command;
      authoritatively or absolutely directive; commanding;
      authoritative; as, imperative orders.

            The suit of kings are imperative.     --Bp. Hall.

   2. Not to be avoided or evaded; obligatory; binding;
      compulsory; as, an imperative duty or order.

   3. (Gram.) Expressive of commund, entreaty, advice, or
      exhortation; as, the imperative mood.

Imperative \Im*per"a*tive\, n. (Gram.)
   The imperative mood; also, a verb in the imperative mood.

Source : WordNet®

imperative
     adj 1: requiring attention or action; "as nuclear weapons
            proliferate, preventing war becomes imperative";
            "requests that grew more and more imperative" [ant: {beseeching}]
     2: relating to verbs in the imperative mood

imperative
     n 1: a mood that expresses an intention to influence the
          listener's behavior [syn: {imperative mood}, {jussive
          mood}]
     2: some duty that is essential and urgent