Strictly speaking, as can be inferred by its etymology, 'microtonal' refers to small intervals. Some theorists hold this to designate only intervals smaller than a semitone (using other terms, such as 'macrotonal', to describe other kinds of non-12-edo intervals), while many others use it to refer to any intervals that deviate from the familiar 12-edo scale, even those which are larger than the semitone -- the extreme case being exemplified by Johnny Reinhard, who states that all tunings are to be considered microtonal.
By December of 1999, several tuning theorists who subscribe to the Yahoo tuning group were attempting to agree on a definition, but no clear consensus emerged, with both of the above extremes as well as all the intermediate positions all being defended by various subscribers.
The author's own view is that anyone with an interest in tuning theory must possess a refined sensibility for perceiving subtle pitch nuances, and so therefore the term 'microtonal' is entirely appropriate to describe their musical and theoretical activities.
Any musical interval less than a 12-tone equally tempered semitone of 100 cents.
Some theorists apply the term only to intervals less than a Limma (90Â¢).
Music employing microtones may be termed microtonal from which the noun microtonality is derived.