A E Diminished (Edim) chord contains the notes E, A#, G. It is built from the diminished formula 1 - ♭3 - ♭5, with E as the root.
Why does Edim sound so unsettled?
Because its fifth is flattened. Every stable chord leans on a perfect fifth for its sense of ground, and a diminished chord removes exactly that. What's left is tension with no resolution built in — which is precisely why it works as a one-beat passing chord and rarely as a destination.