The A flat diminished chord is a 3-note chord consisting of the notes Ab, Cb and Ebb.
You can see these notes highlighted in the interactive piano chart below.
The chord itself is often abbreviated as Abdim.
Chords that are a superset of Abdim. The chords include more notes but always Ab, Cb and Ebb.
In case you prefer a non-interactive variant of the chord chart, we've embedded a PNG image below that shows the notes for the the Abdim chord. Feel free to save or share the image as needed.