$\textit{Activity}$ 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.
Applicable units are those of quantitykind:Activity
http://en.wikipedia.org/wiki/Radioactive_decay
http://en.wikipedia.org/wiki/Radioactive_decay#Radioactive_decay_rates
http://dbpedia.org/resource/Radioactive_decay
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 $A = -\frac{dN}{dt}$.
http://www.iso.org/iso/catalogue_detail?csnumber=31895