concave

American Heritage® Dictionary of the English Language, Fourth Edition

1. adj. Curved like the inner surface of a sphere.
2. n. A concave surface, structure, or line.
3. v. To make concave.

Century Dictionary and Cyclopedia

1. Curved or rounded in the manner of the circumference of a circle or the surface of a sphere when viewed from the center; presenting a hollow or incurvation; incurved; hence, bounded by such a line or surface: as, a concave mirror. A concave bounding surface of a body is one which is so bent that a straight line joining any two points of it lies without the body. Thus, if a ball floats upon water, the common surface of the ball and water is concave if conceived as belonging to the water, and convex if conceived as belonging to the ball. A surface or curve is said to be concave toward the region which would be outside a body of which the curve or surface was a concave boundary.
2. Hollow; empty.
3. n. A hollow; an arch or vault; a concavity.
4. n. Any inwardly curved portion of a machine: as, the concave of a thresher (the curved breast in which the cylinder works).
5. n. A concave mirror.
6. To make hollow.
7. n. The vault or arch of heaven: as, “the blue concave.”
8. n. A playing card so trimmed that it can be separated from others according to the purpose for which the pack was prepared.

Wiktionary

1. adj. curved like the inner surface of a sphere or bowl
2. n. A concave surface or curve.
3. n. The vault of the sky.
4. n. One of the celestial spheres of the Ptolemaic or geocentric model of the world.
5. n. manufacturing An element of a curved grid used to separate desirable material from tailings or chaff in mining and harvesting.
6. n. surfing An indentation running along the base of a surfboard, intended to increase lift.
7. n. skateboarding An indented area on the top of a skateboard, providing a position for foot placement and increasing board strength.
8. v. To render concave, or increase the degree of concavity.

GNU Webster's 1913

1. adj. Hollow and curved or rounded; vaulted; -- said of the interior of a curved surface or line, as of the curve of the of the inner surface of an eggshell, in opposition to convex.
2. adj. rare Hollow; void of contents.
3. n. A hollow; an arched vault; a cavity; a recess.
4. n. (Mech.) A curved sheath or breasting for a revolving cylinder or roll.
5. v. To make hollow or concave.

WordNet 3.0

1. adj. curving inward