con·cave 
(kŏn-kāv, kŏnkāv′)
(kŏn-kāv, kŏnkāv′)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; see keuə- in the Appendix of Indo-European roots.]
con·cavely adv.
con·caveness n.
concaveleft to right: biconcave,
plano-concave, and
concavo-convex lenses