Wiktionary
diminished fifth
n. (context music English) A type of tritone in musical composition comprising of six semitone. 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.
Usage examples of "diminished fifth".
Zaphod screamed a diminished fifth himself, dropped his light and sat heavily on the floor, or rather on a body which had been lying there undisturbed for around six months and which reacted to being sat on by exploding with great violence.