concave left to right: biconcave, plano-concave, and concavo-convex lenses con·cave (kŏn-kāv′, kŏn′kāv′) adj. Curved like the inner surface of a sphere. n. A concave surface, structure, or line. tr.v. con·caved, con·cav·ing, con·caves To make concave. [Middle English, from Latin concavus : com-, intensive pref.; see com- + cavus, hollow ...