Lewis & Short

impĕrātīvus (inp-), a, um, adj. [impero], of or proceeding from a command, commanded: feriae, extraordinary, commanded by a magistrate, Macr. S. 1, 16, § 5 sq.: modus, the imperative, Mart. Cap. 3, § 313, and in the grammarians saep.
Adv.: impĕrātīvē, imperatively, Ulp. Reg. tit. 24, 1.