Definition of onsets:
(n) :
(medicine) The initial phase of a disease or condition, in which symptoms first become apparent.
(n) :
A setting about; a beginning.
(n) :
(phonology) The initial portion of a syllable, preceding the syllable nucleus.
(n) :
(acoustics) The beginning of a musical note or other sound, in which the amplitude rises from zero to an initial peak.
(n) :
(archaic) An attack; an assault especially of an army.
(v) :
(obsolete) To assault; to set upon.