Diminished chord is a chord made by the musical degrees: 1, 3b, 5b, 7bb.
Observation: 7bb is the same as diminished seventh. As 5b is the diminished fifth, in this chord we have two diminished notes. Then, it is not without reason that this chord is called “diminished chord”, is it?!
Learn more here: http://www.simplifyingtheory.com/diminished-chord/