adjective
1.
absolutely necessary or required; unavoidable:
It is imperative that we leave.
2.
of the nature of or expressing a command; commanding.
3.
Grammar. noting or pertaining to the mood of the verb used in commands, requests, etc., as in Listen! Go!
Compare indicative (def 2), subjunctive (def 1).
noun
4.
a command.
5.
something that demands attention or action; an unavoidable obligation or requirement; necessity:
It is an imperative that we help defend friendly nations.
6.
Grammar.
the imperative mood.