Microtonal

[Kai]

Music using microtones; musical intervals significantly smaller than a [half step]. (E.A. quarter [tones] or smaller)
/\ that is the most precise definition, but more generally it can refer to any tuning system other than 12 tone equal temperament, usually not including those that sound like it. (well temperaments)
[Arabic music] doesnt use quarter tones, contrary to popular belief. Arabic music uses neutral intervals. (half way in between major and minor) For example, the neutral second is approximately 3/4 tones.

[Sonia]

Denoting or employing an interval or intervals smaller than a semitone.