A 'tritone' is the name for the musical interval between two pitches an augmented 4th/Diminshed 5th apart. The term 'tritone' comes from the fact that the interval between the two pitches is a full three "whole tones" apart. The use of the tritone is common in classical music where the V-I resolution is present, as well as in Jazz and Rock whenever a piece or section is in the lydian mode. The interval has a very unstable characterisic and was even known as the 'devil's interval' at one period in music history.
Tritones can be heard in the following examples:
-Bass riff of "Jerry Was A Racecar Driver" by Primus
-The first two notes of "Maria" from West Side Story ("ma - ree")
-Opening intervals on "Purple Haze" by Jimi Hendrix
-Opening two notes of the "The Simpsons" theme