noun (n)

• the phenomenon of not permitting the passage of electromagnetic radiation(noun.phenomenon)

  Antonym:
  transparence, transparency
  

  source: wordnet30
• incomprehensibility resulting from obscurity of meaning(noun.attribute)

  Synonym:
  opaqueness
  

  source: wordnet30
• the quality of being opaque to a degree; the degree to which something reduces the passage of light(noun.attribute)

  Antonym:
  clarity, clearness, uncloudedness
  

  source: wordnet30
• The state of being opaque; the quality of a body which renders it impervious to the rays of light; want of transparency; opaqueness.(noun)

  source: webster1913