onset

Short definition from Wiktionary

Noun

1. 1
   The initial phase of a disease or condition, in which symptoms first become apparent.
2. 2
   The initial portion of a syllable, preceding the syllable nucleus.
3. 3
   The beginning of a musical note or other sound, in which the amplitude rises from zero to an initial peak.
4. 4
   A setting about; a beginning.
5. 5
   Anything added, such as an ornament or as a useful appendage.
6. 6
   An attack; an assault especially of an army.

Verb

1. 1
   To assault; to set upon.
2. 2
   To set about; to begin.