diminished fifth

English

diminished fifth (plural diminished fifths)

(music) A type of tritone in musical composition comprising six semitones. It spans five diatonic scale degrees like a perfect fifth but narrower by a semitone. The number of diatonic degrees it spans is what distinguishes it from an augmented fourth.
e.g. C - G is a perfect fifth, C - G♭ is a diminished fifth, and C - G# is an augmented fifth.

Aurally it is impossible to distinguish between an augmented fourth and a diminished fifth. For this reason one would call it a "tritone" in a context such as ear training, but distinguish between an augmented fourth and a diminished fifth when, for example, analysing a score.