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]

Moby Thesaurus II by Grady Ward, 1.0:

81 Moby Thesaurus words for "opacity":
   abstruseness, adiaphanousness, ambiguity, amorphousness, asininity,
   blockishness, boobishness, boorishness, bovinity, cloddishness,
   cloudiness, crassness, crudeness, dark, darkness, denseness,
   density, dim-wittedness, dimness, doltishness, dull-wittedness,
   dullardism, dullness, dumbness, duncery, equivocation, fog,
   fogginess, fuzziness, grossness, hebetude, impenetrability,
   impermeability, imperviousness to light, indefiniteness,
   indeterminateness, indistinctness, intransparency, lethargy,
   loutishness, lumpishness, mist, mistiness, mumbo jumbo, murk,
   murkiness, mystification, ninnyism, oafdom, oafishness,
   obfuscation, obscurantism, obscuration, obscurity, obtuseness,
   opaque, opaqueness, perplexity, reconditeness, roil, roiledness,
   shapelessness, simpletonianism, slowness, sluggishness,
   sottishness, stolidity, stupidity, thick-headedness,
   thick-wittedness, thickness, turbidity, turbidness, unclarity,
   unclearness, unintelligibility, unplainness, unteachability,
   vagueness, wrongheadedness, yokelism