What is an x-ray contrast medium?

Barium, etc..: The most common contrast agents for X-ray include non-iodinated contrast, barium sulfate, and air. Non-iodinated contrast and barium are dense materials which absorb X-rays and allow visualization within a space (vessels, bowel, joints, spine). Air can act as a negative contrast agent, most commonly used together with barium in the colon (double contrast barium enema).

It is a substance: used in medical imaging to help identify and delineate pathology and anatomy. It is called contrast because it has physical properties that make it look dissimilar to the normal tissue appearance(for example, barium absorbs xrays and looks white). Thus, it can outline abnormalities.