diminished fifth

Noun

 * 1)  A type of tritone in musical composition formed by narrowing a perfect fifth by one chromatic semitones. In twelve-tone equal temperament, 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 notes
Aurally, it is impossible to distinguish between an augmented fourth and a diminished fifth in 12-tone equal temperament. 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.

In most other tunings, such as 19-tone, 31-tone, 43-tone, and 53-tone equal temperament, the augmented fourth and diminished fifth are of different sizes.