Source : Webster's Revised Unabridged Dictionary (1913)

Onset \On"set`\, v. t.
   1. To assault; to set upon. [Obs.]

   2. To set about; to begin. [Obs.] --Carew.

Onset \On"set`\, n. [On + set.]
   1. A rushing or setting upon; an attack; an assault; a
      storming; especially, the assault of an army. --Milton.

            The onset and retire Of both your armies. --Shak.

            Who on that day the word of onset gave.
                                                  --Wordsworth.

   2. A setting about; a beginning. [Obs.] --Shak.

            There is surely no greater wisdom than well to time
            the beginnings and onsets of things.  --Bacon.

   3. Anything set on, or added, as an ornament or as a useful
      appendage. [Obs.] --Johnson.

Source : WordNet®

onset
     n 1: the beginning or early stages; "the onset of pneumonia"
          [syn: {oncoming}]
     2: (military) an offensive against an enemy (using weapons);
        "the attack began at dawn" [syn: {attack}, {onslaught}, {onrush}]