Radioactivity is defined as a phenomenon in which certain heavy and unstable atomic nuclei spontaneously disintegrate by emitting certain invisible radiations.
Radioactivity was accidentally discovered by Henri Becquerel when he was investigating the newly discovered X-rays in 1896.
Later on, the Nobel prize-winning couple Marie Sklodowska Curie and Pierre Curie did extensive research on radioactivity and on specific elements such as Radium.