What is meant by the term "activity" in nuclear physics?

The term "activity" in nuclear physics specifically refers to the rate at which a radioactive substance undergoes decay. It is quantitatively defined as the number of decay events that occur in a sample of radioactive material per unit of time, typically expressed in decays per second. This measurement is crucial for understanding the behavior of radioactive materials and is directly related to their stability and the level of radiation they emit.

The correct answer emphasizes the concept of decay, reflecting how "activity" is a measure of the dynamic process of radioactive decay rather than static properties like mass or energy produced. In practical applications, knowing the activity of a material helps in assessing its safety, efficacy in medical treatments, and environmental impact.