concave

Curved like the inner surface of a sphere or bowl.

Not convex; having at least one internal angle greater than 180 degrees.

Satisfying the property that all segments connecting two points on the function's graph lie below the function.

Hollow; empty.

as concave […] as a worm-eaten nut

A concave surface or curve.

The vault of the sky.

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).

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

An indentation running along the base of a surfboard, intended to increase lift.

To render concave, or increase the degree of concavity.