The A diminished seventh chord is a 4-note chord consisting of the notes A, C, Eb and Gb.
You can see these notes highlighted in the interactive piano chart below.
The chord itself is often abbreviated as Adim7.
Chords that are a subset of Adim7. These contain less notes but all of them are included in Adim7.
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 Adim7 chord. Feel free to save or share the image as needed.