Having two leaves or folding parts: as, a bivalve speculum.In zoology, having two shells united by a hinge.In botany, having two valves, as a seed-case.n. plural Folding doors.n. In zoology, a headless lamellibranch mollusk whose shell has two hinged valves, which are opened and shut by appropriate muscles: opposed to univalve.n. In botany, a pericarp in which the seed-case opens or splits into two parts.