Radiation.

Radiation in physics describes any process in which energy emitted by one body travels through a medium or through space, ultimately to be absorbed by another body. Non-physicists often associate the word with ionizing radiation (e.g., as occurring in nuclear weapons, nuclear reactors, and radioactive substances), but it can also refer to electromagnetic radiation such as Radio waves, infrared light, visible light and ultraviolet light, which can also be ionizing radiation. What makes it radiation is that the energy radiates (i.e., it travels outward in straight lines in all directions) from the source. This geometry naturally leads to a system of measurements and physical units that are equally applicable to all types of radiation.

Non-ionizing radiation, by contrast, refers to any type of radiation that does not carry enough energy per quantum to ionize atoms or molecules. Most especially, it refers to the lower energy forms of electromagnetic radiation, radio waves, microwaves infrared and visual light. The effects of these forms of radiation on living tissue have only recently been studied. Instead of producing charged ions when passing through matter, the electromagnetic radiation has sufficient energy only for excitation, the movement of an electron to a higher energy state. Nevertheless, different biological effects are observed for different types of non-ionizing radiation.