Soapwort, in botany: A genus of the digynia order, belonging to the decandra clas of plants; and in the natural method ranking under the 22nd order, Caryophyllaceae. The calyx is monophyllous and naked; there are five unguinated petals; the capsule is oblong and unicellular.

There are eight species, the officinalis, vaccaria, cretica, porrigens, illyrica, ocyoides, orientalis, and lutea. The officinalis, which is a British plant, has a creeping root, so that in a short time it would fill a large space of ground. The stalks are about two feet high, and of a purplish colour. The footstalks of the flowers arise from the wings of the leaves opposite; they fulfill four, five, or more purple flowers each; which have generally two small leaves placed under them. The stalk is also terminated by a loose bunch of flowers growing in form of an umbel; they have each a large swelling cylindrical empalement, and five broad obtuse petals, which spread open, of a purple colour. These are succeeded by oval capsules, with one cell filled with small seeds.—The decoction of this plant is used to cleanse and scour woollen cloths: the poor people in some countries use it instead of soap for washing; from which use it had its name.