The description of the word you requested from the astronomical dictionary is given below.
elektron = [Greek] gold or amber; he Magnetos lithos = [Greek] stone of Magnesia; radiatio = [Latin] radiate
Electromagnetic radiation is any kind of radiation that consists of alternating electric and magnetic fields and that can propagate even in a vacuum. Electromagnetic radiation is characterized by its wavelength (or, equivalently, its frequency or energy). Some different types of electromagnetic radiation or electromagnetic waves, in increasing order of frequency and energy and decreasing order of wavelength, are: radio waves, microwaves, infrared light, visible light, ultraviolet light, X-rays, and gamma rays. In general, the later in the list the type is, the more dangerous it is. Visible light is the only form of electromagnetic radiation that we can see with our eyes.