flavins tightly bound or covalently attached to a protein chain, commonly through the 8-methyl carbon atom.
Source:
PAC, 1995, 67, 1307. 'Glossary of class names of organic compounds and reactivity intermediates based on structure (IUPAC Recommendations 1995)' on page 1335 (https://doi.org/10.1351/pac199567081307)