The C eleventh chord is a 5-note chord consisting of the notes C, G, Bb, D and F.
You can see these notes highlighted in the interactive piano chart below.
The chord itself is often abbreviated as C11.
Chords that are a superset of C11. The chords include more notes but always C, G, Bb, D and F.
Chords that are a subset of C11. These contain less notes but all of them are included in C11.
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 C11 chord. Feel free to save or share the image as needed.