Tritone
Tritone is an interval composed of three tones, so an augmented fourth or a diminished fifth.
Examples of tritones:

A tritone is present in dominant seventh chord in root position and on some inversions of dominant seventh chord.
dominant seventh chord in root position:

Resolution of tritone
In tonal music, the tritone is dissonant and must be resolved:

Note:
In tonal music,
- A diminished fifth should be resolved with a smaller interval (third)
- An augmented fourth should be resolved with a bigger interval (sixth)
The tritone, the devil’s interval
The tritone is also called devil’s interval in western music because of its dissonant characteristic.