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From 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]