Science, Tech, Math › Science Effusion Definition Chemistry Glossary Definition of Effusion Share Flipboard Email Print This diagram shows effusion (left) and diffusion (right). Effusion occurs through a hole smaller than the mean path of moving particles while diffusion occurs through a hole large enough allow multiple particles to pass at once. Astrang13, Creative Common License 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 07, 2017 Effusion Definition: Effusion is the movement of a gas through a pore or capillary into another gaseous region or into a vacuum.