adjective

1. having the nature of, or indicating, power or authority; commanding an imperative gesture
2. absolutely necessary; urgent; compelling it is imperative that I go
3. Gram. designating or of the mood of a verb that expresses a command, strong request, or exhortation

Etymology: LL imperativus, commanding < pp. of L imperare, to command: see emperor

noun

1. a binding or compelling rule, duty, requirement, etc.
2. a command; order
3. Gram.
   1. the imperative mood
   2. a verb in this mood