A tritone is an interval spanning three whole tones or, alternatively, spanning six half tones. It is between a perfect fourth and a perfect fifth, making it a diminished fifth and an augmented fourth, and it is exactly half an octave. The prefix “tri” refers to the three whole steps in between the two notes.
The tritone's main application is to create a feeling of tension, which is then resolved by moving to the adjacent chord in the chord progression. It is often perceived as having a gloomy and unsettling sound and used to be referred to as the "devil's interval." Once regarded as something to be avoided, it has become common in popular music, especially in jazz.