A substance or object that is opaque is neither transparent nor translucent. It does not allow light to pass through it. The property of being opaque is called opacity. Degrees of opacity are converse to degrees of transparency.

In science the phrase "an opacity" may be used to describe an opaque object.

Opacity is often used as a metaphor, for example in describing written and spoken language. Here opaque means confusing or unintelligible. In politics, governmental decision-making that is not conducted publicly may be called opaque. (See Transparency (humanities).)