Science, Tech, Math › Science Phase Diagram Definition What Is a Phase Diagram? Share Flipboard Email Print This is a phase diagram, which includes the critical point and triple point. Booyabazooka, Wikipedia Commons 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 12, 2019 Phase Diagram Definition A phase diagram is a chart showing the thermodynamic conditions of a substance at different pressures and temperatures. The regions around the lines show the phase of the substance and the lines show where the phases are in equilibrium. Parts of a Phase Diagram Typically, a phase diagram includes lines of equilibrium or phase boundaries. On these lines, multiple phases of matter can exist at equilibrium. The lines also indicate where phase transition occur. Triple points occur where lines of equilibrium intersect. A triple point identifies the condition at which three phases of matter can coexist. The temperature below which a substance forms a stable solid is called the solidus. The temperature above which a substance forms a stable liquid is the liquidus.