Opaque means impossible or very difficult to see through. A frosted window is opaque: you know there's light on the other side, but you can't make out any details. A brick wall is completely opaque. Regular glass is transparent (clear), but if you paint it or frost it, it becomes opaque.

The word also describes things that are hard to understand, like confusing instructions or unclear explanations. When a teacher gives opaque directions for an assignment, students leave class confused about what they're supposed to do. Government documents are often criticized for being opaque when they use complicated language that hides what they're really saying.

In science, opacity (the noun form) matters a lot. Astronomers study how opaque different clouds of gas are to determine what they're made of. Doctors choose materials for certain medical tools based on whether they need to be opaque or transparent.

The opposite of opaque is transparent. Between these extremes lies translucent, which lets some light through but blurs the details, like wax paper or a shower curtain. Understanding these three words helps you describe exactly how much you can or cannot see through something.