concave

Short definition from Wiktionary

Adjective

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

Noun

1. 1
   A concave surface or curve.
2. 2
   The vault of the sky.
3. 3
   One of the celestial spheres of the Ptolemaic or geocentric model of the world.
4. 4
   An element of a curved grid used to separate desirable material from tailings or chaff in mining and harvesting.
5. 5
   An indentation running along the base of a surfboard, intended to increase lift.
6. 6
   An indented area on the top of a skateboard, providing a position for foot placement and increasing board strength.
7. 7
   A playing card made concave for use in cheating.

Verb

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