[opac.i.ty] n, pl -ties [F opacite shadiness, fr. L opacitat-, opacitas, fr. opacus shaded, dark] (ca. 1611) 1: the quality or state of a body that makes it impervious to the rays of light; broadly: the relative capacity of matter to obstruct the transmission of radiant energy
2. a: obscurity of sense: unintelligibleness b: the quality or state of being mentally obtuse: dullness
3: an opaque spot in a normally transparent structure (as the lens of the eye)