"Microtonal"/"microtone" is a term closely related to xenharmonic music that sometimes generates some confusion.
Dictionaries define microtonal music as music that employs intervals smaller than a semitone.
Sometime before 1900, composer Julián Carrillo Trujillo performed experiments on a violin string, using a razor blade to achieve very precise intervals smaller than a semitone, which he called "microtono." Over a decade later, the music theorist Maud MacCarthy Mann began using the term "microtone" to describe Indian sruti intervals that were smaller than a semitone, to differentiate them from quarter tones. In the 1910's and 1920's, there was some discussion as to whether the term was appropriate, or if competing terms, such as "heterotone" or "fraction-tone" etc., would be clearer. By the 1930's, with interest in American Blues music booming, and with people like Ivor Darreg becoming active with new tuning methods, many more terms were proposed, but the terms "microtonal" and "xenharmonic" were most prominent in the English language by the end of the decade.
Generally, in colloquial use, "microtonal music" is any music that isn't 12edo, even if it is in a tuning system that does not use any intervals less than a 12edo semitone step.