- a metastable microconstituent of any of various forms of carbon steel, produced by undercooling sufficiently below the normal transformation temperature, especially a hard, brittle product of the decomposition of austenite, produced in this way.
- a constituent formed in steels by rapid quenching, consisting of a supersaturated solid solution of carbon in iron. It is formed by the breakdown of austenite when the rate of cooling is large enough to prevent pearlite forming
