The Collaborative International Dictionary of English v.0.48:

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.
      [1913 Webster]

            The onset and retire
            Of both your armies.                  --Shak.
      [1913 Webster]

            Who on that day the word of onset gave.
                                                  --Wordsworth.
      [1913 Webster]

   2. A setting about; a beginning; -- used especially of
      diseases or pathological symptoms. --Shak.
      [1913 Webster +PJC]

            There is surely no greater wisdom than well to time
            the beginnings and onsets of things.  --Bacon.
      [1913 Webster]

   3. Anything set on, or added, as an ornament or as a useful
      appendage. [Obs.] --Johnson.
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Onset \On"set`\, v. t.
   1. To assault; to set upon. [Obs.]
      [1913 Webster]

   2. To set about; to begin. [Obs.] --Carew.
      [1913 Webster]

WordNet (r) 3.0 (2006):

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

Moby Thesaurus II by Grady Ward, 1.0:

147 Moby Thesaurus words for "onset":
   A, aggravated assault, aggression, albertype, alpha,
   amphibious attack, appearance, armed assault, assailing,
   assailment, assault, attack, banzai attack, beginning, birth,
   blast-off, blitz, blitzkrieg, book printing, breakthrough, charge,
   chromotypography, chromotypy, chromoxylography, collotype,
   color printing, commencement, counterattack, counteroffensive,
   coup de main, creation, crippling attack, cutting edge, dawn,
   dawning, dead set at, debut, descent on, diversion,
   diversionary attack, drive, edge, electronography,
   electrostatic printing, establishment, flank attack, flying start,
   foundation, fresh start, frontal attack, gas attack, genesis,
   graphic arts, gravure, halftone engraving, head-on attack,
   history of printing, hit, inauguration, inception, infiltration,
   initiation, institution, job printing, jump-off, kick-off,
   leading edge, letterpress, letterpress photoengraving,
   lightning attack, lightning war, line engraving, lithography,
   lithogravure, lithophotogravure, mass attack, megadeath,
   mimeograph, mugging, new departure, offense, offensive, offset,
   offset lithography, oncoming, onrush, onslaught, opening, origin,
   origination, outbreak, outset, outstart, overkill,
   palaeotypography, panzer warfare, photo-offset,
   photochemical process, photoengraving, photogelatin process,
   photographic reproduction, photography, photolithography,
   phototypography, phototypy, photozincography,
   planographic printing, planography, printing, printmaking,
   publication, publishing, push, raid, relief printing,
   rotary photogravure, rotogravure, run against, run at,
   running start, rush, sally, send-off, setout, setting in motion,
   setting-up, sheetwork, shock tactics, sortie, square one, start,
   start-off, starting point, stencil, storming, strike, take-off,
   three-color printing, two-color printing, typography,
   typolithography, unprovoked assault, wood-block printing,
   xerography, xeroprinting, xylotypography, zincography



U.S. Gazetteer Places (2000):

Onset, MA -- U.S. Census Designated Place in Massachusetts
   Population (2000):    1292
   Housing Units (2000): 910
   Land area (2000):     1.083253 sq. miles (2.805612 sq. km)
   Water area (2000):    0.215025 sq. miles (0.556913 sq. km)
   Total area (2000):    1.298278 sq. miles (3.362525 sq. km)
   FIPS code:            51160
   Located within:       Massachusetts (MA), FIPS 25
   Location:             41.746424 N, 70.663251 W
   ZIP Codes (1990):     02532
   Note: some ZIP codes may be omitted esp. for suburbs.
   Headwords:
    Onset, MA
    Onset