The C altered chord is a 4-note chord consisting of the notes C, E, Bb and Db.
You can see these notes highlighted in the interactive piano chart below.
The chord itself is often abbreviated as Calt7.
Chords that are a superset of Calt7. The chords include more notes but always C, E, Bb and Db.
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 Calt7 chord. Feel free to save or share the image as needed.