Not allowing light to pass through; not transparent or translucent. Figuratively, it means difficult to understand or lacking clarity.
From Latin opacus meaning 'dark, shady', possibly related to opus 'work' suggesting something that blocks or works against light. The figurative sense of unclear or obscure developed in the 17th century.
Materials can be opaque to visible light but transparent to other wavelengths - your skin is opaque to visible light but transparent to X-rays, which is how medical imaging works. In computer graphics, opacity levels are measured as percentages, with 100% being completely opaque and 0% being completely transparent.
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