Science, Tech, Math › Science Essential Amino Acid Definition Share Flipboard Email Print Methionine is an essential amino acid. Todd Helmenstine 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. 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. Updated March 06, 2017 Definition: An essential amino acid is an amino acid which an organism needs to ingest because it is necessary for nutrition and cannot be synthesized in the body. Also Known As: indispensable amino acid Examples: Isoleucine and leucine are examples of amino acids which are essential amino acids in humans. Continue Reading