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Opacity (optics), describes opaque material, impenetrable to visible light.

In politics, governmental decision-making that is not conducted publicly may be called opaque. See Transparency (humanities)

In the humanities, opacity is a metaphor used in describing written or spoken language. Here "opaque" means difficult to understand or unseen. See Transparency (linguistic)

Opaque context, a term used in philosophical theories of reference.

In computer software, functions or data are opaque if their internal mechanisms or data structures cannot be accessed or utilised. This has been found to be advantageous in that it allows the internals of routines and their data structures to be freely modified without affecting programs that utilize those routines.

"Opacity" and "opaque" are used in various contexts to describe something that is not transparent.

"opaque" is a special flag on many Unix Filesystems