concave

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.
7. A playing card made concave for use in cheating.

Plural concaves