III. Concave·noun A hollow; an arched vault; a cavity; a recess.
IV. Concave·noun A curved sheath or breasting for a revolving cylinder or roll.
V. Concave·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; as, a concave mirror; the concave arch of the sky.