Castor oil \Cas"tor oil\ (k[a^]s"t[~e]r oil`). A mild cathartic oil, expressed or extracted from the seeds of the {Ricinus communis}, or {Palma Christi}. When fresh the oil is inodorous and insipid. {Castor-oil plant}. Same as {Palma Christi}.
castor oil n : a purgative extracted from the seed of the castor plant; used in paint and varnish as well as medically