Definition: An interval of three whole tones (Augmented 4th / Diminished 5th). Historically known as the “Diabolus in musica” (Devil in music) due to its instability.
The Savvy Take: The Tritone cuts the octave exactly in half. It is the most unstable interval and demands resolution. It is the defining sound of the Dominant 7th chord (the distance between 3rd and 7th).