The C diminished chord is a 3-note chord consisting of the notes C, Eb and Gb.
You can see these notes highlighted in the interactive piano chart below.
The chord itself is often abbreviated as Cdim.
Chords that are a superset of Cdim. The chords include more notes but always C, Eb and Gb.
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 Cdim chord. Feel free to save or share the image as needed.