Imperative, a.
Etymology: [L. imperativus, fr. imperare to command; pref. im- in + parare to make ready, prepare: cf. F. impératif. See Perade, and cf. Empire.]
1. Expressive of command; containing positive command; authoritatively or absolutely directive; commanding; authoritative; as, imperative orders. The suit of kings are imperative. Bp. Hall.
2. Not to be avoided or evaded; obligatory; binding; compulsory; as, an imperative duty or order.
3. (Gram.)
Expressive of commund, entreaty, advice, or exhortation; as, the imperative mood.
1 other Webster entry under this spelling
Imperative, n. (Gram.)
The imperative mood; also, a verb in the imperative mood.
Shades of meaning
Fernald has no entry of its own for imperative; he lists it under absolute.
Where he writes about it:
Authoritative is used always in a good sense, implying the right to claim authority; imperative, peremptory, and positive are used ordinarily in the good sense; as, an authoritative definition; an imperative demand; a peremptory command; positive instructions; imperious signifies assuming and determined to command, rigorously requiring obedience. — from absolute