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