Nuclide is a substance composed of the same atoms determined by the proton number Z (i.e. by number of protons - electrically positively charged structural elements of atom's nucleus) and by the nucleon number A (i.e. total number of protons and neutrons - neutral structural elements of atom's nucleus). Radionuclide is a radioactive nuclide, i.e. nuclide with nuclei subject to radioactive changes.

Radiation alpha, beta and gamma.
They are three types of radiation, which are emitted at radioactive changes. Particles alpha are, in reality, helium nuclei composed of two protons and two neutrons, emitted most frequently in the course of disintegration of heavy nuclei. Electrons or positrons emitted in the course of disintegration of atom's nucleus are called beta particles. Radiation gamma differs from radiation alpha and beta by the fact that it is not a stream of particles but it is electromagnetic radiation emitted as a result of different processes in atom's nucleus.