An SI (Système International d'Unités) International System of Units of radioactivity, equal to one radioactive disintegration (change in he nucleus of an atom when a particle or ray is given off) per second or equal to the activity resulting from the decay of one nucleus of radioactive matter in one second or other nuclear transformation per second.

Named after the French physicist, Antoine Henri Becquerel, who discovered that rays emitted by uranium salts affect photographic plates (1852-1908).