The A dominant ninth chord is a 5-note chord consisting of the notes A, C#, E, G and B.
You can see these notes highlighted in the interactive piano chart below.
The chord itself is often abbreviated as A9.
Chords that are a superset of A9. The chords include more notes but always A, C#, E, G and B.
Chords that are a subset of A9. These contain less notes but all of them are included in A9.
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 A9 chord. Feel free to save or share the image as needed.