An Oxymel traditionally is a blend of honey, water, and vinegar, formerly used for medicinal purposes. In meadmaking, an oxymel is a mead mixed with vinegar (usually wine vinegar) after fermentation. It is not clear whether oxymel was ever made with mead originally, or if this is a modern innovation and medicinal versions were mixed with pure honey.
Oxymels were not intended for recreational drinking and are a very unusual style among modern meadmakers.