FamousMu ChordSteely Dan · Reelin' in the Years (1972)
Not one moment: a chord type used on every Steely Dan album. Named examples are the intro of Reelin' in the Years (1972, root-position type) and Deacon Blues (1977, with the third in the bass).
A major chord with an added second placed right next to the third, so the two notes rub a whole step apart inside an otherwise sweet chord. Steely Dan's Walter Becker and Donald Fagen called it the mu major and used it on every album, from the intro of Reelin' in the Years (1972) onward. It stays clearly major but sounds richer and slightly smudged, without turning into a jazz seventh chord.
Also called Mu major, Steely Dan chord, μ chord, μ major, add2 chord voiced with the second just below the third
Sources: Mu chord (Wikipedia) · Intro to the Steely Dan Song Book (sdarchive.com) · The Mu Chord (Steely Dan Chord) for Piano (Piano With Jonny)