Definition of: Prussian blue

1. Chem. Any one of a group of cyanogen compounds formed from ferrous sulfate and potassium ferrocyanide: formerly much used in dyeing.
2. A deep, strong, blue pigment with a coppery sheen, obtained from these compounds: used in oil painting but impermanent on alkali surfaces, as fresco: also called Paris blue (formerly called Berlin blue). Heat changes it to Prussian brown. [So called because discovered accidentally in Berlin, 1704, by H. de Diesbach, a colormaker]