Merriam-Webster Collegiate Dictionary - concave

 I. adjective  Etymology: Middle English, from Latin concavus, from com- + cavus hollow — more at cave  Date: 15th century  1. hollowed or rounded inward like the inside of a bowl  2. arched in ; curving in — used of the side of a curve or surface on which neighboring normals to the curve or surface converge and on which lies the chord joining two neighboring points of the curve or surface  II. noun  Date: 1552 a ~ line or surface