* Radiopaque: This term means "not allowing radiation to pass through". In the context of X-rays, it refers to substances that block or absorb X-ray beams.

* Radiolucent: The opposite of radiopaque. These substances allow X-rays to pass through relatively easily.

Examples of radiopaque substances:

* Bones: Contain calcium, which is a good absorber of X-rays. This is why you can see bones clearly in an X-ray image.

* Metal: Metals are also excellent absorbers of X-rays.

* Barium: A contrast agent often used in medical imaging to highlight organs like the digestive system.

In summary, a radiopaque substance is one that absorbs X-rays, preventing them from reaching the X-ray detector and creating a shadow on the image.