The term for the time it takes for half of a radioactive substance to decay is called "half-life." This concept is crucial in nuclear physics and radiometric dating, where the half-life describes how long it takes for half of a given amount of a radioactive isotope to transform into a different element or isotope. Different isotopes have varying half-lives, ranging from fractions of a second to billions of years. Understanding half-lives helps scientists assess the age of rocks, fossils, and archaeological finds, playing a significant role in fields such as geology and anthropology.