hydrocyanic acid

A colourless liquid or gas, HCN, with a characteristic odour of almonds; r.d. 0.699 (liquid at 22°C); m.p. –14°C; b.p. 26°C. It is an extremely poisonous substance formed by the action of acids on metal cyanides. Industrially, it is made by catalytic oxidation of ammonia and methane with air and is used in producing acrylate plastics. Hydrogen cyanide is a weak acid (K_a=2.1 × 10⁻⁹ mol dm⁻³). With organic carbonyl compounds it forms cyanohydrins.

An aqueous solution of hydrogen cyanide that is a poisonous weak acid. Also called prussic acid.

A volatile liquid that is extremely poisonous because it checks the oxidation process in protoplasm. See also cyanide.

The noun has one meaning:

Meaning #1: a weak poisonous acid (HCN) used in fumigating and in synthesis of organic compounds
  Synonyms: hydrogen cyanide, Prussic acid