The non-notatable (In Western terms) variances in rhythmic performance; microtiming is essentially the miniscule timing deviants from an established metric periodicity.
Microtiming takes one of several identities in musical literature:
“Expressive” – the individual idiosyncrasies in a performance, including patterns of expression and spontaneous irregularities.
“Structural” – the systematic variations that generally pervade all musical performance (e.g. the lengthening of the last event in group).
“Stylistic” – the variation applied by practitioners of a particular musical style or genre (e.g., swing ratio, backbeat).
“Human Factor” – the random variations without which performance is generally experienced as mechanical.
(Citations – NYUAD?)