The Lydian mode is a medieval mode starting on the fourth degree of the diatonic scale, with half steps (semitones) between the fourth and fifth and seventh and eighth degrees. The Lydian mode is a major scale with a raised fourth. It is been described as bitter-sweet in character: the sweetness coming from the major third, the darker aspect from the sharpened fourth.
The Lydian mode is often used for atmospheric music in films and is a favorite of guitarist Steve Vai. It is used in the open tune of The Simpsons.