Lydian is the 4th mode of the major scale. In the key of C major we get F Lydian. The notes of Lydian are the same as its related major scale, however the lydian mode has an augmented 4th, also referred to as a #11.

Despite the root chord of the mode being a major 7, the lydian mode also has a tense and mysterious quality due to the #11. Since there is a whole-tone between the 3rd and 4th notes we get a semitone between the 4th and 5th.