Linguistics The part of a syllable that precedes the nucleus. In the word nucleus (nōō'klē-əs), the onset of the first syllable is (n), the onset of the second syllable is (kl), and the last syllable has no onset.
1535, "attack, assault," from on + set (n.); cf. to set (something) on (someone). Weaker sense of "beginning, start" first recorded 1561. Fig. use in ref. to a calamity, disease, etc. is from c.1586.
[on-set, awn-] 
