Term: metastable https://doi.org/10.1351/goldbook.M03872 Definition: Related Terms: 1) metastable ion (<em>in mass spectrometry</em>) (http://doi.org/10.1351/goldbook.M03874). 2) metastable state (<em>in mass spectrometry</em>) (<em>in nuclear chemistry</em>) (http://doi.org/10.1351/goldbook.M03875). 3) metastable state (<em>in mass spectrometry</em>) (<em>in nuclear chemistry</em>) (<em>in spectrochemistry</em>) (http://doi.org/10.1351/goldbook.M03876). 4) transient (chemical) species (<em>in mass spectrometry</em>) (<em>in nuclear chemistry</em>) (<em>in spectrochemistry</em>) (http://doi.org/10.1351/goldbook.T06451). Source: PAC, 1972, 31, 577. 'Manual of Symbols and Terminology for Physicochemical Quantities and Units, Appendix II: Definitions, Terminology and Symbols in Colloid and Surface Chemistry' on page 609 (https://doi.org/10.1351/pac197231040577) Citation: 'metastable' in IUPAC Compendium of Chemical Terminology, 5th ed. International Union of Pure and Applied Chemistry; 2025. Online version 5.0.0, 2025. https://doi.org/10.1351/goldbook.M03872 License: The IUPAC Gold Book is licensed under Creative Commons Attribution-ShareAlike CC BY-SA 4.0 International (https://creativecommons.org/licenses/by-sa/4.0/) for individual terms. Disclaimer: The International Union of Pure and Applied Chemistry (IUPAC) is continuously reviewing and, where needed, updating terms in the Compendium of Chemical Terminology (the IUPAC Gold Book). Users of these terms are encouraged to include the version of a term with its use and to check regularly for updates to term definitions that you are using.