Abmaj11 is Ab, C, Eb, G, Bb and Db — with the eleventh added — the note that fights the major third.
It sounds unsettled, and grittier than the rest of the major family. Most players drop the third or raise the eleventh instead, which is how you arrive at a sus chord or at maj7♯11.
Tonic, awkwardly — the eleventh sits a semitone above the third.
Also written AbM11, AbΔ11 or Ab major 11.

How you get to it. The turn that lands on Abmaj11 is to — the ii and the V of Ab — and shares its third and seventh with , so the hand barely moves between them.
Hear it land:→→
In which keys. Every note of Abmaj11 belongs to F minor (the ♭III) and Ab major (the I, the tonic).
Every note of Abmaj11 is in Ab major.
Each of those is a page: the scale across the neck, and the chords it harmonises into — which is where Abmaj11 sits in it. Every scale.
Change one note of Abmaj11 and you have (drop the eleventh), (the eleventh becomes the thirteenth) or (the major third becomes the minor third).
Abmaj11 is a major chord — root, major third, fifth. Major chords on every root.
The common shapes and the derived voicings come from different places, and both are explained in how this site is built.
Shows each chord the way this style would voice it, and marks which of its alternatives belong to the idiom.