hydrogen cyanide

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A colorless, volatile, extremely poisonous flammable liquid, HCN, miscible in water and used in the manufacture of dyes, fumigants, and plastics. It has an almondlike odor and becomes a gas at 26°C.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun inorganic chemistry A colourless, very poisonous, volatile liquid, HCN, used in the production of dyes, plastics and fumigants; it dissolves in water to form hydrocyanic acid and reacts with bases to form cyanides, and with some organic compounds to form nitriles.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun a highly poisonous gas or volatile liquid that smells like bitter almonds; becomes a gas at around 90 degree Fahrenheit and is most dangerous when inhaled; the anhydride of hydrocyanic acid; used in manufacturing