Radiodensity is the property of relative transparency to the passage of X-rays through a material. Radiolucent indicates greater transparency to X-Ray photons. Radiodense indicates greater opacity to X-Ray photons.
X-Rays are part of the electromagnetic spectrum, with photon energies above visible (and ultraviolet) light photons. They are distinguished from gamma rays in that they are produced not by transitions within the atomic nucleus, but either by decceleration of a charged particle or by the transition of state of orbital electrons. Diagnostic x-rays are produced using X-Ray tubes.
The radio waves portion of the electromagnetic spectrum represent much lower energy/frequency photons than visible light. Thus, referring to the property of X-Ray density as radiodensity appears contradictory by current knowledge, but is still used as a historical artifact.
Though radiodensity is commonly used in a qualitative comparison perspective, it can also be quantified according to the Hounsfield scale, which is fundamental to CT scanning applications. Specifically, distilled water is specified on the Hounsfield scale, as a reference, as 0 units ("HU", or Hounsfield units), air is specified as -1000 HU. These were chosen as universally available references and were oriented to the key application CT was developed for: imaging the internal anatomy of living creatures based on organized water structures and mostly living in air, e.g. humans.