semitone. Half a tone. Smallest interval in European mus. On pf., interval between any note and the next, up or down. There are three types of semitone: diatonic (same as a minor 2nd); chromatic (difference between major 2nd and minor 2nd); and enharmonic (doubly diminished 3rd). Diatonic semitone has ratio of 16/15, and two chromatic semitone intervals are recognized (‘lesser semitone’ of 25/24 and ‘greater semitone’ of 135/128). These intervals occur between values of chromatic-scale notes conventionally accepted as ‘just’ or ‘perfect’.

sem·i·tone / ˈsemēˌtōn; ˈsemˌī-/ • n. Mus. the smallest interval used in classical Western music, equal to a twelfth of an octave or half a tone; a half step.