The time required for half of a radioactive isotope's original concentration to decay to a more stable form is called its:

1. Radioactive isotopes decay at a characteristic, predictable rate, which is described using a specific time-based term. 2. The text defines it directly: "The time it takes for half of the original concentration of an isotope to decay back to its more stable form is called its half-life." 3. This concept underlies radiometric dating techniques, such as using carbon-14 to date formerly living material. 4. So this decay-rate concept is called half-life, distinct from atomic mass, mass number, or oxidation state. _Source: OpenStax Biology Ch 2 "The Chemical Foundation of Life", p.35 §2.1 | Atoms, Isotopes, Ions, and Molecules: The Building Blocks_