concave

adj

1. Curved like the inner surface of a sphere or bowl.
2. (not-comparable)Not convex; having at least one internal angle greater than 180 degrees.
3. (not-comparable)Satisfying the property that all segments connecting two points on the function's graph lie below the function.
4. Hollow; empty.

   “as concave […] as a worm-eaten nut”

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

verb

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