An interval measures the distance between two pitches, defined by both a number (second, third, fourth, etc.) and a quality (major, minor, perfect, augmented, or diminished). A major third spans four semitones, while a minor third spans three. Perfect intervals (unison, fourth, fifth, octave) have a uniquely stable quality.
Intervals are the building blocks of both melody and harmony. Learning to identify intervals by ear is a fundamental skill in music training — many musicians associate specific intervals with the openings of well-known songs. A perfect fifth sounds like Twinkle Twinkle Little Star, while a minor second is the Jaws theme.