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.
不透明 is the na-adjective meaning 'opaque', while 不透明性 is the noun form referring to the property of opacity itself.
透明性 means 'transparency', the opposite of 不透明性.
Compound of 不 (fu, 'not') + 透明 (tōmei, 'transparent') + 性 (sei, '-ness, -ity'), literally 'non-transparency-ness'.