me·nis·cus / məˈniskəs/ • n. (pl. -ci / -kē; -kī/ or -cus·es) Physics the curved upper surface of a liquid in a tube. ∎ Optics [usu. as adj.] a lens that is convex on one side and concave on the other. ∎ Anat. a thin fibrous cartilage between the surfaces of some joints, e.g., the knee.