n. In zoöl., anat., and medicine: The organ of attachment of a vertebrate embryo or fetus to the wall of the uterus or womb of the female.n. In echinoderms, a flat discoidal sea-urchin, as a sand-dollar or cake-urchin: used in a generic sense by Klein, 1734.n. [capitalized] A genus of bivalve mollusks, now called Plaruna.n. In botany, that part of the ovary of flowering plants which bears the ovules.