concave

Etymology

From Middle English concave, from Old French concave, from Latin concavus.

adj

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

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. (manufacturing) An element of a curved grid used to separate desirable material from tailings or chaff in mining and harvesting.
5. (surfing) An indentation running along the base of a surfboard, intended to increase lift.
6. (skateboarding) An indented area on the top of a skateboard, providing a position for foot placement and increasing board strength.
7. (gambling) A playing card made concave for use in cheating.
   Coordinate term: convex

verb

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