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- n. A chord in musical composition consisting originally of the notes F, B, D# and G# which can be translated more generally in intervalic terms as: augmented fourth, augmented sixth and augmented ninth above a root. Enharmonically it sounds like a half-diminished seventh chord (e.g. F-A♭-C♭-E♭), though in terms of musical analysis it can be interpreted in several ways.
It is a reference to the chord used in the opening phrase of Richard Wagner's opera Tristan and Isolde as part of the leitmotif attributed to the character Tristan. (Wiktionary)