$\textit{Activity}$ is the number of decays per unit time of a radioactive sample, the term used to characterise the number of nuclei which disintegrate in a radioactive substance per unit time. Activity is usually measured in Becquerels ($Bq$), where 1 $Bq$ is 1 disintegration per second, in honor of the scientist Henri Becquerel.
Variation $dN$ of number of nuclei $N$ in a particular energy state, in a sample of a radionuclide, due to spontaneous nuclear transitions from this state during an infinitesimal time interval, divided by its duration $dt$; thus: activity $A = -\frac{dN}{dt}$.
Applicable units are those of quantitykind:Activity
Activity