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 photons with sufficient energy (ionizing photons), but other times is 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. For astronomy purposes, 10 eV is within the order-of-magnitude of enough to ionize atomic hydrogen from its ground state and would ionize any hydrogen atom above that 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.