The A altered chord is a 4-note chord consisting of the notes A, C#, G and Bb.
You can see these notes highlighted in the interactive piano chart below.
The chord itself is often abbreviated as Aalt7.
Chords that are a superset of Aalt7. The chords include more notes but always A, C#, G and Bb.
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 Aalt7 chord. Feel free to save or share the image as needed.