Noun

1. A concave surface or curve.
2. The vault of the sky.
3. One of the celestial spheres of the Ptolemaic or geocentric model of the world.

   Aristotle makes [Fire] to move to the concave of the Moon. - Thomas Salusbury (1661).

4. An element of a curved grid used to separate desirable material from tailings or chaff in mining and harvesting.
5. An indentation running along the base of a surfboard, intended to increase lift.
6. An indented area on the top of a skateboard, providing a position for foot placement and increasing board strength.

Verb

1. To render concave, or increase the degree of concavity.