1the interval which is a semitone less than a minor seventh, e.g. from A to G flat (which in equal tuning sounds the same as a major sixth).
2 (also diminished seventh chord) a chord formed by a note together with those above it at intervals of a minor third, a diminished fifth, and a diminished seventh. The resulting chord consists entirely of superimposed minor thirds, and is much used in modern music in modulating between keys.
diminished seventh in other Oxford dictionaries
Definition of diminished seventh in the US English dictionary