any of several species of plants belonging to the genus Rubia of the madder family, Rubiaceae. Rubia tinctorum and R. peregrina are native European plants, and R. cordifolia is native to the hilly districts of India and Java. Rubia is a genus of about 60 species; its members are characterized by lance-shaped leaves that grow in whorls and by small yellowish flowers that grow in clusters.

The common madder (R. tinctorum) and R. cordifolia were formerly cultivated for a red dye, alizarin, that was obtained from the ground-up roots of these plants. This dye was used for cloth and could be prepared and applied in such a way as to yield pink and purple shades as well as red. The dye properties of the madder root appear to have been known from the earliest historical times; cloth dyed with madder has been found on ancient Egyptian mummies, and madder was used for dying the cloaks of Libyan women in the time of Herodotus (5th century BC). Madder was also employed as a medicinal treatment for amenorrhea (failure to menstruate) in ancient and medieval times. Another property of alizarin is that it colours red the bones of animals that feed upon madder. This property was used by 19th-century physiologists to trace the way in which bone develops and to study the functions of the various types of cells in growing bone. In the 1860s a way was found to manufacture alizarin synthetically, and so the once-extensive use of madder as a source of alizarin dye has now practically disappeared.