activity,  nuclear    The  rate  of  disintegration  (dN/dt)of  a  radioactive  sample. The  SI unit for activity is  Becquerel (symbol Bq).

       1 Bq = 1 decay/s

More  commonly  curie  (symbol  Ci)  is  used as a unit for nuclear activity.

       1 Ci = 3.7 x 10¹⁰ decays/s.

It is number of disintegrations per second in one gram of  radium.