im·per·a·tive 
(ĭm-pĕrə-tĭv)
(ĭm-pĕrə-tĭv)adj.
1. Necessary or urgent: "It is imperative that we continue to move with speed to make housing more affordable" (Timothy Geithner). See Synonyms at urgent.
2. Expressing a command or plea; peremptory: requests that grew more and more imperative.
3. Grammar Of, relating to, or constituting the mood that expresses a command or request.
n.
1.
a. A rule, principle, or need that requires or compels certain action: "the internal tension in [military] doctrine, between the desire to prescribe a common way of fighting and the imperative of adjusting particular military actions to circumstances" (Eliot A. Cohen).
b. A command; an order.
2. Grammar
a. The imperative mood.
b. A verb form of the imperative mood.
[Middle English imperatif, relating to the imperative mood, from Old French, from Late Latin imperātīvus, from Latin imperātus, past participle of imperāre, to command; see EMPEROR.]
im·pera·tive·ly adv.
im·pera·tive·ness n.