Adjective
Plane or flat on one side, and concave on the other; as, a plano-concave lens. See Lens.
Source: Webster's dictionaryIf the lens is biconcave or plano-concave, a collimated beam of light passing through the lens is diverged (spread); the lens is thus called a negative or diverging 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