CITRUS, in botany, a genus of the polyadelphia icosandria class. The calix is divided into five segments; the petals are five, and oblong; and the fruit is a berry, consisting of nine cells. The species are three, viz. the medica, or lemon-tree, a native of Asia; the aurantium, or orange-tree, a native of the Indies; and the trifoliata, a native of Japan.