Science, Tech, Math › Science Definition of Air in Science Share Flipboard Email Print Kayocci/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 November 29, 2019 The term "air" refers generally to gas, but exactly which gas depends on the context in which the term is used. Let's learn about the modern definition of air in scientific disciplines and the earlier definition of the term. Modern Air Definition Air is the general name for the mixture of gases that makes up the Earth's atmosphere. This gas is primarily nitrogen (78%), mixed with oxygen (21%), water vapor (variable), argon (0.9%), carbon dioxide (0.04%), and trace gases. Pure air has no discernible scent and no color. Air typically contains dust, pollen, and spores; other contaminants are referred to as "air pollution." On another planet—Mars, for example—the so-called air would have a different composition since there is technically no air in space. Older Air Definition Air is also an early chemical term for a type of gas. In the older definition, many individual types of so-called air made up the air we breathe: Vital air was later determined to be oxygen; what was called phlogisticated air turned out to be nitrogen. An alchemist might refer to any gas released by a chemical reaction as its "air."