Ionizing radiation is radiation with sufficient energy to ionize neutral atoms or compounds. The term is used regarding the effect of electromagnetic radiation (EMR) on human safety, and is sometimes used in astronomy and planetary science. Sometimes it is meant to refer to EMR with sufficient energy photons, but also is sometimes meant to include particles with analogous kinetic energy, such as cosmic rays and products of radioactivity.
10 eV is sometimes cited as the low-end limit, e.g., for safety purposes, and for astronomy purposes, that is in the order-of-magnitude to ionize atomic hydrogen, and would do so for any above its ground state. Given the great variety of compounds and the possible states of excitation of atoms and molecules that may require little energy to free an electron, there is no straight-forward cut-off. At high photon energy (thousands or millions times the 10 eV minimum), the freed electron has enough energy to trigger many additional ionizations.
Given that minimum, it includes the short-wavelength end of ultraviolet and all of X-rays and gamma rays. Early stars (e.g., OB stars) produce such radiation, which is a factor in their effect on surrounding clouds, e.g., the Strömgren sphere model. active galactic nuclei also produce such energetic photons and affect surrounding interstellar medium.