What is "nitroprussic"

Nitroprussic \Ni`tro*prus"sic\ (? or ?), a. [Nitro- + prussic.] (Chem.) Pertaining to, derived from, or designating, a complex acid called nitroprussic acid, obtained indirectly by the action of nitric acid on potassium ferrocyanide (yellow prussiate), as a red crystalline unstable substance. It forms salts called nitroprussides, which give a rich purple color with alkaline sulphides.