DIADELPHIA, (dia, twice, and adelphos, a brother,) class the 17th in the sexual system, comprehending those plants which bear hermaphrodite flowers with two sets of united stamens, but this circumstance must not be absolutely depended on. They are the papilionacei of Tournefort, the irregularis tetrapetali of Rivinus, and the leguminosa of Ray. See BOTANY, p. 1292, and Plate LIX.