imperative

Concise Oxford English Dictionary © 2008 Oxford University Press:
imperative/ɪmˈperətɪv/

▶adjective

• 1 of vital importance.

• 2 giving an authoritative command.

  ■ Grammar denoting the mood of a verb that expresses a command or exhortation, as in come here!

▶noun an essential or urgent thing.

■ a factor that makes something necessary.

– derivatives
imperatival /ɪmˌpɛrəˈtʌɪv(ə)l/ adjective,
imperatively adverb,
imperativeness noun.

– origin ME: from late L. imperativus (lit. ‘specially ordered’), from imperare ‘to command’, from in- ‘towards’ + parare ‘make ready’.