Activity is the term used to characterise the number of nuclei which disintegrate in a radioactive substance per unit time usually measured in Becquerels (Bq); a Bq is 1 disintegration per second. Replaces the former unit curie, symbol: Ci. 1 Ci is equal to 37,000,000,000 Bq.

Note: “Activity” is a quantitative term whereas “radioactivity” is a qualitative term used to describe atoms that decay.