Science, Tech, Math › Science What Is the Definition of a Fluid? Chemistry Glossary Definition of Fluid Share Flipboard Email Print anton5146 / 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 August 12, 2019 A fluid is any substance that flows or deforms under applied shear stress. Fluids comprise a subset of the states of matter and include liquids, gases, and plasma. Examples All liquids and gases are fluids (air, water, oil).