Broadly speaking, radiation is an emanation of energy, the most notable example being the electromagnetic spectrum. However, in more common practice, it refers to any part of the EM spectrum that is not visible light and to be more specific, only refers to hazardous ionizing radiation, which also includes subatomic particles in addition to the upper parts of the EM spectrum.
In media, it is often stereotypically depicted as green gas or ooze. A more realistic depiction would be a blue or purple haze/glow around the emitting object, to represent the ionization of the surrounding air and/or Cherenkov radiation caused by extremely radioactive objects or film grain from the exposure of radiation on a camera lens. An artistic representation may be stylized "rays" coming from this object.