microtonal

/ˌmaɪkroʊˈtoʊnəl/
adjective
  1. Relating to or using musical intervals smaller than a half step (semitone), common in some non-Western music and modern experimental music.
    • Some traditional Indian music is microtonal, with many subtle pitch variations.
    • The composer wrote a microtonal piece that used quarter tones instead of the usual piano notes.
    • The guitarist experimented with a microtonal tuning to create unusual sounds.