Radiation; radio waves.
[The first element of English radiation (from Latin radiat-, emitted in rays), plus -o-.]
The core sense is that of electromagnetic radiation, such as heat, light, X-rays, and gamma rays. Terms with this sense include radiometer, an instrument for detecting or measuring the intensity or force of radiation; radiology, the science dealing with X-rays and other high-energy radiation, especially its use for the diagnosis and treatment of disease; and radiotherapy, the treatment of disease, especially cancer, using X-rays or similar forms of radiation.
The form is most widely used today in an extended sense in which it refers to the emission of subatomic particles or ionizing radiation from atomic nuclei, as in the adjective radioactive, referring to a substance that emits them; a radioisotope is a radioactive isotope of an atom; radiochemistry is the branch of chemistry concerned with radioactive substances.
A narrower sense refers to one form of electromagnetic radiation, radio waves (radio as a standalone word is an abbreviation of terms like radio-telephony, in which radio- has the broader sense of radiation); examples are often self-explanatory (radio-controlled, radiotelescope) and some are now dated (radiolocation, an old term for radar).