Originally: a metasoma. Later also: the segment of the body of copepods and certain other crustaceans that is situated between the cephalothoracic portion (cephalosome) and the urosome, anterior to the body's major articulation.
Originally: a mineral or rock formed by metasomatism (now rare). Later: a metasomatized layer of the upper mantle.
Late 19th century; earliest use found in Ogilvie's Imperial Dictionary. From meta- + -some, after metasoma.