cyan(o)-

The colour blue, especially dark blue; cyanide or a derivative.

Greek kuan(e)os, dark blue.

The poisonous gas cyanogen (C₂N₂), though itself colourless, was named in 1815 by the French chemist Joseph Gay-Lussac (1778–1850) because he found it was chemically related to the dark-blue dyestuff Prussian Blue. The group —CN is now called the cyano group. Inorganic or mineral compounds containing it are cyanides (organic salts are nitriles). Cyanoacrylates, cyanide derivatives of acrylates, are quick-setting adhesives.

The colour cyan is a light greenish-blue, one of the three principal secondary colours (the others being magenta and yellow), complementary to red. Cyanosis is a blueness of the skin owing to poor circulation or inadequate oxygenation of the blood. A cyanobacterium is the modern name for what used to be called a blue-green alga; it is a bacterium capable of photosynthesis.