What is "diminished sixth"

n. (context music English) an interval that is equivalent to a perfect fifth

In classical music from Western culture, a diminished sixth is an interval produced by narrowing a minor sixth by a chromatic semitone. For example, the interval from A to F is a minor sixth, eight semitones wide, and both the intervals from A to F, and from A to F are diminished sixths, spanning seven semitones. Being diminished, it is considered a dissonant interval.

Its inversion is the augmented third, and its enharmonic equivalent is the perfect fifth.