| opacification | (noun) the process of becoming cloudy or opaque | - |
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| opacity | (noun) the quality of being opaque to a degree; the degree to which something reduces the passage of light | Synonyms: opaqueness |
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| (noun) incomprehensibility resulting from obscurity of meaning | Synonyms: opaqueness |
| (noun) the phenomenon of not permitting the passage of electromagnetic radiation | - |
| opah | (noun) large elliptical brightly colored deep-sea fish of Atlantic and Pacific and Mediterranean | Synonyms: Lampris regius, moonfish |
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| opal | (noun) a translucent mineral consisting of hydrated silica of variable color; some varieties are used as gemstones | - |
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| opalescence | (noun) the visual property of something having a milky brightness and a play of colors from the surface | Synonyms: iridescence |
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| opaqueness | (noun) the quality of being opaque to a degree; the degree to which something reduces the passage of light | Synonyms: opacity |
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| (noun) incomprehensibility resulting from obscurity of meaning | Synonyms: opacity |