名詞 imperative:

1. a mood that expresses an intention to influence the listener's behavior
   同義詞：imperative mood, jussive mood, imperative form
2. some duty that is essential and urgent

形容詞 imperative:

1. requiring attention or action
2. relating to verbs in the imperative mood

1. It's imperative that you apologize to him immediately.
   你必須立即向他道歉。
2. Job creation has become an imperative for the government.
   創(chuàng)造就業(yè)機會是政府必須做的事。
3. Prompt action is imperative.
   果斷的行動是必要的。

im.per.a.tive

adj.Abbr. imp.（形容詞）縮寫 imp.
Expressing a command or plea; peremptory:
專橫的：表示命令或懇求的；不許違反的：
例句：
requests that grew more and more imperative.
越來越強制的要求

Having the power or authority to command or control.
強制的：有權(quán)力或權(quán)威去命令或控制的
Grammar Of, relating to, or constituting the mood that expresses a command or request.
【語法】 祈使的：表示或構(gòu)成命令或要求語氣的
Impossible to deter or evade; pressing:
必要的，緊急的：不可能防止或逃避的；緊急的：
例句：
imperative needs.See Synonyms at urgent
緊急的需要參見 urgent

n.Abbr. imp.（名詞）縮寫 imp.

A command; an order.
命令，要求
An obligation; a duty:
責(zé)任；義務(wù)：
例句：
social imperatives.
社會職責(zé)

A rule, a principle, or an instinct that compels a certain behavior:
驅(qū)使：迫使采取某一行為的規(guī)則、原則或本能：
例句：
a people driven to aggression by territorial imperatives.
為擴張領(lǐng)土的意念所驅(qū)而侵略成性的民族

Grammar
【語法】
The imperative mood.
祈使語氣
A verb form of the imperative mood.
祈使語氣動詞

來源：
Middle English imperatif [relating to the imperative mood]
中古英語 imperatif [和祈使語氣有關(guān)的]
from Old French
源自 古法語
from Late Latin imper3tovus
源自 后期拉丁語 imper3tovus
from Latin imper3tus [past participle of] imper3re [to command] * see emperor
源自 拉丁語 imper3tus [] imper3re的過去分詞 [命令] *參見 emperor

【引伸】
imper.atively
adv.（副詞）
imper.ativeness
n.（名詞）