Definition of "concave"

Curved like the inner surface of a sphere or bowl.  examples

(geometry, not comparable, of a polygon) Not convex; having at least one internal angle greater than 180 degrees.  examples

(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.  examples

Hollow; empty.  quotations  examples

A concave surface or curve.  examples

The vault of the sky.  examples

One of the celestial spheres of the Ptolemaic or geocentric model of the world.  examples

(manufacturing) An element of a curved grid used to separate desirable material from tailings or chaff in mining and harvesting.  examples

(surfing) An indentation running along the base of a surfboard, intended to increase lift.  examples

(skateboarding) An indented area on the top of a skateboard, providing a position for foot placement and increasing board strength.  examples

(gambling) A playing card made concave for use in cheating.  examples

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