concave

a.

  1. 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; as, a concave mirror; the concave arch of the sky.
  2. Hollow; void of contents.

n.

  1. A hollow; an arched vault; a cavity; a recess.
  2. A curved sheath or breasting for a revolving cylinder or roll.

v. t.

  1. To make hollow or concave.