Source : Webster's Revised Unabridged Dictionary (1913)

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.

   2. Obscurity; want of clearness. --Bp. Hall.

Source : WordNet®

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