Radiopaque: Opaque to one or another form of radiation, such as X-rays. Radiopaque objects block radiation rather than allow it to pass through. Metal, for instance, is radiopaque, so metal objects that a patient may have swallowed are visible on X-rays. Radiopaque dyes are used in radiology to enhance X-ray pictures of internal anatomic structures. The opposite of radiopaque is radiolucent.