- a princess in medieval legend; known as Iseult the Fair. She was the daughter of the king of Ireland. Tristram was sent to seek her hand on behalf of his uncle, King Mark of Cornwall. En route back to Cornwall Tristram and Iseult mistakenly drank the love potion intended for the wedding night of Iseult and Mark and fell hopelessly in love. Iseult was bound to marry King Mark, but she and Tristram continued their affair until Mark became suspicious and Tristram had to flee. The tragic story is part of the body of Arthurian literature. Versions of the legend were told by Thomas of Brittany (fl. 12th century), Gottfried von Strassburg (fl. 12th-13th century), Alfred, Lord Tennyson, and Richard Wagner in his opera Tristan.