The C diminished seventh chord is a 4-note chord consisting of the notes C, Eb, Gb and Bbb.
You can see these notes highlighted in the interactive piano chart below.
The chord itself is often abbreviated as Cdim7.
Chords that are a subset of Cdim7. These contain less notes but all of them are included in Cdim7.
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 Cdim7 chord. Feel free to save or share the image as needed.