imperative

Short definition from Wiktionary

Adjective

1. 1
   Essential; crucial; extremely important.
2. 2
   Of, or relating to the imperative mood.
3. 3
   Having semantics that incorporates mutable variables.
4. 4
   Expressing a command; authoritatively or absolutely directive.

Noun

1. 1
   The grammatical mood expressing an order (see jussive). In English, the imperative form of a verb is the same as that of the bare infinitive.
2. 2
   A verb in the imperative mood.
3. 3
   An essential action, a must: something which is imperative.