The word "opaque" refers to a substance or material that does not allow light to pass through it. In other words, if something is opaque, you cannot see through it. This term is often used to describe things like frosted glass, thick paints, or even ideas and arguments that are difficult to understand.
The term "opaque" comes from the Latin word "opacus," which means "shaded" or "dark." The transition to modern English includes influences from French, where it is used similarly to describe something that blocks light.