Concave

Concave ( " excavated " from the Latin concavus, " inwardly curved " ) refers to:

An inward curve, see curvature
Negative sign of the radius of curvature in the curve discussion, see convex and concave functions
Inwardly curved lens surfaces, see Lens (optics) # Verschiedene_Linsenformen
Inwardly curved mirror surfaces, see concave mirror