Source : Webster's Revised Unabridged Dictionary (1913)

Concavity \Con*cav"i*ty\, n.; pl. {Concavities}. [L. concavitas:
   cf. F. concavit['e]. See {Concave}.]
   A concave surface, or the space bounded by it; the state of
   being concave.

Source : WordNet®

concavity
     n 1: a shape that curves or bends inward [syn: {concave shape}, {incurvation},
           {incurvature}]
     2: the property possessed by a concave shape [syn: {concaveness}]