Ionization

Ionization
Process by which electrically neutral atoms or molecules are converted to electrically charged atoms or molecules (ions) by the removal or addition of negatively charged electrons.

It is one of the principal ways in which radiation transfers energy to matter, and hence of detecting radiation. In general, ionization occurs whenever sufficiently energetic charged particles or radiant energy travels through gases, liquids, or solids. A certain minimal level of ionization is present in the earth's atmosphere because of continuous absorption of cosmic rays from space and ultraviolet radiation from the sun.
Source:Britannica encyclopedia