augmented fourth
noun
°(music) A type of tritone in musical composition comprising of six semitones. 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.
"e.g. C - F is a perfect fourth, C - F# is an augmented fourth, and C - F♭ is a diminished fourth."