- The definition of hydrocyanic acid is a colorless, very poisonous liquid with the smell of bitter almonds.
An example of hydrocyanic acid is cyanide.
a weak, highly poisonous acid, HCN, that is a colorless liquid with the odor of bitter almonds: it is used as a fumigant, poison gas, in metallurgy, etc.
An aqueous solution of hydrogen cyanide. Also called prussic acid.