The Collaborative International Dictionary of English v.0.48:

Opacity \O*pac"i*ty\, n. [L. opacitas: cf. F. opacit['e].]
   1. The state of being opaque; the quality of a body which
      renders it impervious to the rays of light; want of
      transparency; opaqueness.
      [1913 Webster]

   2. Obscurity; want of clearness. --Bp. Hall.
      [1913 Webster]

WordNet (r) 3.0 (2006):

opacity
    n 1: the phenomenon of not permitting the passage of
         electromagnetic radiation [ant: transparence,
         transparency]
    2: incomprehensibility resulting from obscurity of meaning [syn:
       opacity, opaqueness]
    3: the quality of being opaque to a degree; the degree to which
       something reduces the passage of light [syn: opacity,
       opaqueness] [ant: clarity, clearness, uncloudedness]