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 doesn't 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.
Dude! Arabic music is sick, with all those quarter tones and stuff.
Arabic music doesn't use quarter tones, the smallest interval used is a half step; Arabic music isn't really even microtonal. Go listen to some Greek enharmonic scales or Charles Ives!