concave

/ˈkɒŋkeɪv/

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.

1
Curved like the inner surface of a sphere or bowl
2
(of a polygon) not convex; having at least one internal angle greater than 180 degrees.
3
(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

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