Abmaj7b5 is Ab, C, Ebb and G. It keeps the major 3rd and 7th of , but lowers the 5th one fret to the flat 5th (Ebb).
The flat 5th replaces the natural 5th. A maj7 sharp 11 chord keeps the natural 5th and adds the sharp 11. The names are not interchangeable.
Keep the flat 5th audible. Without it, the remaining notes only spell .
Also written AbM7b5, AbΔ7b5 or Abmaj7(b5).

How you get to it. The turn that lands on Abmaj7b5 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 Abmaj7b5 belongs to C minor (the ♭VI) and Eb major (the IV, the subdominant).
Contains Abmaj7b5 and adds ninth, fifth and thirteenth.
Contains Abmaj7b5 and adds ninth, sharp fifth and thirteenth.
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.