Adjective
Plane or flat on one side, and convex on the other; as, a plano-convex lens. See Convex, and Lens.
Source: Webster's dictionaryIf the lens is biconvex or plano-convex, a collimated beam of light passing through the lens converges to a spot (a focus) behind the lens. Source: Internet
If one of the surfaces is flat, the lens is plano-convex or plano-concave depending on the curvature of the other surface. Source: Internet