The becquerel (Bq) is the SI unit of radioactivity, serving as the standard measure for the rate of decay of a radioactive substance. It quantifies the amount of radioactivity by defining one becquerel as one nuclear decay or disintegration per second. This unit indicates the activity of a sample, reflecting the number of decay events occurring at a specific point in time. Measuring radioactivity using this scale is fundamental in fields such as nuclear physics, medicine, and environmental science to assess the natural or induced decay of atomic nuclei.