An Ionizing radiation is any type of radiation capable of producing ionization in materials it contacts; includes high-energy charged particles such as alpha and beta rays, and nonparticulate radiation such as gamma rays and X-rays. In contrast to wave radiation (e.g., visible light and radio waves) in which waves do not ionize adjacent atoms as they move.
Other /More definition:
An ionizing radiation is Emissions from naturally occurring and man-made radionuclides, including alpha, beta, and gamma emissions.