Definition from Wiktionary, the free dictionary
From French opacité.
- (uncountable) The state or quality of being opaque, not allowing light to pass through
- We could not see the sandbar due to the opacity of the muddy water.
- (uncountable) The state or quality of being inaccessible to understanding
- The opacity of these decisions is troubling.
- (optics, countable) A measure of relative impenetrability to electromagnetic radiation such as light.
- The opacity of a clean glass window is near zero.
Coordinate terms 
the state of being opaque
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