DIADELPHIA (δις, "twice," and αδελφης, "a brother"), the 17th class 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 irregulariter tetrapetali of Rivinus, and the leguminosae of Ray. See BOTANY INDEX.