Webster's English Dictionary

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opacity

opac.i.ty \o--'pas-*t-e-\ n [F opacite` shadiness, fr. L opacitat-, 
   opacitas, fr. opa]cus shaded, dark 1a: the quality or state of a body that 
   makes it impervious to the rays of  light : OPAQUENESS 1b: the capacity of 
   matter to obstruct the transmission of forms of radiant  energy in addition 
   to light 2a: obscurity of sense : UNINTELLIGIBLENESS  2b: mental obtuseness 
    3: an opaque spot on a normally transparent structure