Term: absorption coefficient https://doi.org/10.1351/goldbook.A00037 Definition: Linear decadic (a,K) and Napierian absorption coefficients (α) are equal to the corresponding absorbances divided by the optical path length through the sample. The molar absorption coefficients (decadic ɛ, Napierian κ) are the linear absorption coefficients divided by the amount concentration. Related Terms: 1) absorbances (http://doi.org/10.1351/goldbook.A00028). 2) amount concentration (http://doi.org/10.1351/goldbook.A00295). Source: Green Book, 2nd ed., p. 32 (https://goldbook.iupac.org/files/pdf/green_book_2ed.pdf) Citation: 'absorption coefficient' 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.A00037 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.