The 7♯11 chord is a dominant 7th with a raised eleventh (augmented fourth) — the signature chord of the Lydian Dominant sound. It creates a bright, sophisticated tension that is central to modern jazz harmony and tritone substitutions.
Why ♯11 instead of natural 11? A natural 11th (perfect 4th) clashes badly with the major 3rd — they are only a semitone apart. The ♯11 avoids this clash entirely, sitting comfortably alongside the third. This is why jazz musicians strongly prefer 7♯11 over 7sus or dom11 for dominant chords. The matching scale is Lydian Dominant (4th mode of melodic minor).