The Collaborative International Dictionary of English v.0.48:

Absorptivity \Ab`sorp*tiv"i*ty\, n.
   Absorptiveness.
   [1913 Webster]

WordNet (r) 3.0 (2006):

absorptivity
    n 1: (physics) the property of a body that determines the
         fraction of the incident radiation or sound flux absorbed
         or absorbable by the body [syn: absorptivity, absorption
         factor]