What is the term for the time it takes for half of a radioactive substance to decay?

The term that describes the time it takes for half of a radioactive substance to decay is known as half-life. This concept is crucial in understanding radioactive decay and the stability of isotopes. During one half-life, the amount of the radioactive isotope reduces to half of its original quantity, which allows scientists to easily calculate the age of materials and the time it takes for a substance to undergo decay. This principle is fundamental in fields like geology, archaeology, and radiometric dating, as it helps determine the age of fossils and geological formations.

The other terms listed, while related to radioactivity, do not specifically denote the duration required for half of a radioactive material to decay. For instance, the decay constant refers to the probability of decay of an individual atom, and the decay period does not represent an established scientific term. Radioactive age could imply the age of a sample based on its radioactive content but is not the correct terminology for the decay process itself.