The Collaborative International Dictionary of English v.0.48:

ionizing radiation \ionizing radiation\ n.
   rays of rapidly moving subatomic particles, x-reys, or gamma
   rays, sufficiently energetic to cause ionization when
   absorbed by matter.

   Note: Such radiation is emitted by radioactive elements, or
         may be generated by highly energetic physical
         processes, as in stars. When absorbed by living tissue,
         ionizing radiation can cause mutation, cellular damage,
         or death.
         [PJC]

WordNet (r) 3.0 (2006):

ionizing radiation
    n 1: high-energy radiation capable of producing ionization in
         substances through which it passes