It's the tritonus, and is a dissonant sound.
"Tritone (tritonus), the interval of the augmented fourth or diminished fifth (also called the tritonic fourth and tritonic fifth), which spans three (whole) tones. The augmented fourth (IV+) and diminished fifth (V°) are enharmonic spellings for the Tritone. These two intervals are the same sound, but they are written differently."
[link to thelemicmusick.net]