Refers to the quality of being opaque, either physically (not allowing light to pass through) or figuratively (lack of transparency in processes or communication).
The opacity of this glass is high.
The opacity of the government is being criticized.
Compound of 不 (fu, 'not') + 透明 (tōmei, 'transparent') + 性 (sei, '-ness, -ity'), literally 'non-transparency-ness'.