Wiktionary
augmented fourth
n. (context music English) A type of tritone in musical composition comprising of six semitone. It spans four diatonic scale degrees like a perfect fourth but with an extension of a semitone. The number of diatonic degrees it spans is what distinguishes it from a diminished fifth.
Usage examples of "augmented fourth".
The Gypsy scale -- minor third, augmented fourth, minor sixth, and major seventh -- fretted my nerves.