What is "diminished octave"

n. (context music English) A musical interval of the Western twelve-semitone system consisting of eleven semitone and spanning eight degrees of the diatonic scale. It is one semitone narrower than a perfect octave and enharmonically equivalent to a major seventh.

In classical music from Western culture, a diminished octave is an interval produced by narrowing a perfect octave by a chromatic semitone. As such, the two notes are denoted by the same letter but have different accidentals. For instance, the interval from C4 to C5 is a perfect octave, twelve semitones wide, and both the intervals from C4 to C5, and from C4 to C5 are diminished octaves, spanning eleven semitones. Being diminished, it is considered a dissonant interval.

The diminished octave is enharmonically equivalent to the major seventh.