Science, Tech, Math › Science Glass Transition Temperature Definition Chemistry Definition of Glass Transition Temperature Share Flipboard Email Print 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 March 06, 2017 Definition of Glass Transition Temperature: The glass transition temperature is the temperature at which an amorphous solid becomes soft upon heating or brittle upon cooling. The glass transition temperature is lower than the melting point of its crystalline form, if it has one.