a. 1. Of or pertaining to, or employing, or operated by, radiant energy, specifically that of electromagnetic waves with frequencies between those of infrared radiation and X-rays; hence, pertaining to, or employed in, broadcast radio or television, microwaves, radiotelephones, etc.; as, radio waves. [ Webster 1913 Suppl. +PJC ]

2. of or pertaining to broadcast radio; as, a radio program. [ PJC ]

A combining form indicating connection with, or relation to, a radius or ray; specifically (Anat.), with the radius of the forearm; as, radio-ulnar, radio-muscular, radio-carpal. [ 1913 Webster ]

a. [ Radio- + active. ] 1. (Physics) Capable of luminescence under the action of cathode rays, X rays, or any of the allied forms of radiation. [ obsolete ] [ Webster 1913 Suppl. ]

2. (Physics) of, exhibiting, or caused by radioactivity. [ PJC ]

n. (Physics) a form of instability which is a property of the atomic nuclei of certain isotopes, which causes a spontaneous change in the structure of the nucleus, accompanied by emission of energetic radiation. The radiation emitted is usually sufficient to cause ionization in matter through which it passes, and is therefore called ionizing radiation. The radiation emitted by most radioactive substances is one of three types: alpha rays, beta rays, or gamma rays. Some chemical elements have no stable isotopes, and these are referred to as radioactive elements, and the element itself is said to possess radioactivity. The changes in radioactive nuclei which cause radiation in most cases cause the chemical identity of the nucleus itself to change, as when tritium (an isotope of hydrogen) emits a beta ray and converts to helium. The radioactive decay process is a first-order reaction, and the rate of decay of a particular isotope can therefore be expressed as the half life of the isotope, which is the time it takes for one half of the remaining undecayed isotope to decay, and is a constant independent of the proportion of original material which has already decayed. The half life of tritium, for example, is 12.3 years. [ PJC ]

n. (Elec.) A substance or device that has its conductivity altered in some way by electric waves, as a coherer. [ Webster 1913 Suppl. ]

‖n. pl. [ NL. See Radiate, and Flagellata. ] (Zool.) A group of Protozoa having both flagella and pseudopodia. [ 1913 Webster ]

n. [ Radio- + -graph. ] 1. An instrument for measuring and recording solar radiation. [ Webster 1913 Suppl. ]

2. An image or picture produced upon a sensitive surface, as of a photographic or fluorescent plate, by some form of penetrating radiation other than light, as X-rays, beta rays, etc.; esp., a picture of the internal structure of opaque objects traversed by the rays; a skiagraph. When the picture is produced upon photographic film by X-rays, the picture is usually called an X-ray photo or X-ray. When an image is produced on photographic film by a radioactive substance in close proximity to the film, in a manner so as to record the spatial distribution of the radioactive substance, the resulting image is called an autoradiograph or autoradiogram. [ Webster 1913 Suppl. +PJC ]

v. t. To make a radiograph of. -- ra`di*og"ra*pher n. [Webster 1913 Suppl.]

n. Art or process of making radiographs, radiograms, or autoradiograms. -- ra`di*o*graph"ic ra`di*o*graph"ic*al a. -- Ra`di*o*graph"ic*al*ly, adv. [Webster 1913 Suppl.]

‖n. pl. [ NL. See Radioli. ] (Zool.) Order of rhizopods, usually having a siliceous skeleton, or shell, and sometimes radiating spicules. The pseudopodia project from the body like rays. It includes the polycystines. See Polycystina. [ 1913 Webster ]