In music, the inclusive distance between one tone and another, whether sounded successively (melodic interval) or simultaneously (harmonic interval).
In Western music, intervals are generally named according to the number of scale-steps within a given key that they embrace; thus, the ascent from C to G (CDEFG) is called a fifth because the interval embraces five scale degrees. There are four perfect intervals: prime, or unison; octave; fourth; and fifth. The other intervals (seconds, thirds, sixths, sevenths) have major and minor forms that differ in size by a half step (semitone). Both perfect and major intervals may be augmented, or enlarged by a half tone. Perfect and minor intervals may be diminished, or narrowed by a half tone.