The A diminished chord is a 3-note chord consisting of the notes A, C and Eb.
You can see these notes highlighted in the interactive piano chart below.
The chord itself is often abbreviated as Adim.
Chords that are a superset of Adim. The chords include more notes but always A, C and Eb.
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 Adim chord. Feel free to save or share the image as needed.