The time it takes for half of a substance to decay is called its "half-life." This term is commonly used in the context of radioactive decay, where unstable isotopes lose half of their atoms at a predictable rate. Each radioactive isotope has a specific half-life, ranging from fractions of a second to billions of years. For example, carbon-14 has a half-life of approximately 5,730 years, which is useful in dating ancient organic materials. Understanding half-lives is crucial in fields such as nuclear physics, geology, and medicine, particularly in the context of radioactive isotopes and their applications.