The unit of measure for radioactivity is the Becquerel (Bq). One Becquerel is defined as one decay event per second and is used to quantify the activity of a radioactive substance. Other units, such as the Curie (Ci), may also be used, where one Curie equals 37 billion disintegrations per second. Measuring radioactivity is essential in fields like nuclear energy, medicine, and environmental science to assess safety and health risks associated with exposure to radioactive materials.