The A eleventh chord is a 5-note chord consisting of the notes A, E, G, B and D.
You can see these notes highlighted in the interactive piano chart below.
The chord itself is often abbreviated as A11.
Chords that are a superset of A11. The chords include more notes but always A, E, G, B and D.
Chords that are a subset of A11. These contain less notes but all of them are included in A11.
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 A11 chord. Feel free to save or share the image as needed.