The harmonic minor scale (or Aeolian ♮7 scale) is a musical scale derived from the natural minor scale, with the minor seventh degree raised by one semitone to a major seventh, creating an augmented second between the sixth and seventh degrees.
Thus, a harmonic minor scale is represented by the following notation:
In classical music from Western culture, a diminished seventh () is an interval produced by narrowing a minor seventh by a chromatic semitone, and its inversion is the augmented second. For instance, the interval from A to G is a minor seventh, ten semitones wide, and both the intervals from A♯ to G, and from A to G♭ are diminished sevenths, spanning nine semitones. Being diminished, it is considered a dissonant interval.
The diminished seventh is used quite readily in the minor key, where it is present in the harmonic minor scale between the seventh scale step and the sixth scale step in the octave above.
View the full Wikipedia page for Diminished seventh