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