Ionizing radiation, or hard radiation, is a form of radiation which carries sufficient energy to ionize an atom or molecule. It can be both natural or artificially generated. Background radiation was determined by measuring the amount of ionizing radiation in a particular location.
Ionizing radiation had the ability to interfere with a starship's sensors. In 2370, Geordi La Forge suggested that the asteroid containing the USS Pegasus be blanketed in ionizing radiation, which also was found naturally in the same area, in order to prevent its discovery by Romulans. (TNG: "The Pegasus")
When Geordi La Forge was investigating the mysterious disappearances of his former crewmates from the USS Victory on Tarchannen III in 2367, he performed a screen for ionizing radiation, without success. (TNG: "Identity Crisis")