Compounds containing an enediol structure stabilized by conjugation and hydrogen bonding with an adjacent carbonyl group,
RC(OH)=C(OH)C(=O)R. They are strong reducing agents, fairly strong acids and commonly derived from saccharides by oxidation at the carbon atom alpha to the carbonyl function, e.g. ascorbic acid.
Source:
PAC, 1995, 67, 1307. 'Glossary of class names of organic compounds and reactivity intermediates based on structure (IUPAC Recommendations 1995)' on page 1363 (https://doi.org/10.1351/pac199567081307)