Science, Tech, Math › Science Half-Life Definition Chemistry Glossary Definition of Half-Life Share Flipboard Email Print An element's half-life is one way to gauge its radioactivity. Jutta Kuss,/fStop Images/Getty Images Science Chemistry Chemical Laws Basics Molecules Periodic Table Projects & Experiments Scientific Method Biochemistry Physical Chemistry Medical Chemistry Chemistry In Everyday Life Famous Chemists Activities for Kids Abbreviations & Acronyms Biology Physics Geology Astronomy Weather & Climate By Anne Marie Helmenstine, Ph.D. Chemistry Expert Ph.D., Biomedical Sciences, University of Tennessee at Knoxville B.A., Physics and Mathematics, Hastings College Dr. Helmenstine holds a Ph.D. in biomedical sciences and is a science writer, educator, and consultant. She has taught science courses at the high school, college, and graduate levels. our editorial process Facebook Facebook Twitter Twitter Anne Marie Helmenstine, Ph.D. Updated February 02, 2019 Half-Life Definition: The time required to convert one half of a reactant to product. The term is commonly applied to radioactive decay, where the reactant is the parent isotope and the product is a daughter isotope.