1.  onset(n = noun.event) oncoming - the beginning or early stages; "the onset of pneumonia"
is a kind of start

2.  onset(n = noun.act) attack, onrush, onslaught - (military) an offensive against an enemy (using weapons); "the attack began at dawn"
is a kind of military operation, operation
has parts: assault
has particulars: ground attack, charge, banzai attack, banzai charge, diversion, diversionary attack, incursion, penetration, blitz, blitzkrieg, strike, counterattack, countermove, bombardment, bombing, fire, firing, strafe, coup de main, surprise attack

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

2.  A setting about; a beginning; -- used especially of diseases or pathological symptoms. Shak. [1913 Webster]
"There is surely no greater wisdom than well to time the beginnings and onsets of things." [1913 Webster]

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

1.  To assault; to set upon. [1913 Webster]

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

onset, n.
1 an attack.
2 a beginning, esp. an energetic or determined one.

N  beginning, commencement, opening, outset, incipience, inception, inchoation, introduction, alpha, initial, inauguration, debut, le premier pas, embarcation, rising of the curtain, maiden speech, outbreak, onset, brunt, initiative, move, first move, narrow end of the wedge, thin end of the wedge, fresh start, new departure, origin, source, rise, bud, germ, egg, rudiment, genesis, primogenesis, birth, nativity, cradle, infancy, start, inception, creation, starting point, dawn, evolution, title-page, head, heading, van, caption, fatihah, entrance, entry, inlet, orifice, mouth, chops, lips, porch, portal, portico, propylon, door, gate, gateway, postern, wicket, threshold, vestibule, propylaeum, skirts, border, first stage, first blush, first glance, first impression, first sight, rudiments, elements, outlines, grammar, alphabet, ABC, beginning, initial, initiatory, initiative, inceptive, introductory, incipient, proemial, inaugural, inchoate, inchoative, embryonic, rudimental, primogenial, primeval, primitive, primordial, aboriginal, natal, nascent, first, foremost, leading, maiden, begun, just begun, at the beginning, in the beginning first, in the first place, imprimis, first and foremost, in limine, in the bud, in embryo, in its infancy, from the beginning, from its birth, ab initio, ab ovo, ab incunabilis, ab origine, let's get going!, let's get this show on the road!, up and at 'em!, aller Anfang ist schwer, dimidium facti qui coepit habet, omnium rerum principia parva sunt.

N  attack, assault, assault and battery, onset, onslaught, charge, aggression, offense, incursion, inroad, invasion, irruption, outbreak, estrapade, ruade, coupe de main, sally, sortie, camisade, raid, foray, run at, run against, dead set at, storm, storming, boarding, escalade, siege, investment, obsession, bombardment, cannonade, fire, volley, platoon fire, file fire, fusillade, sharpshooting, broadside, raking fire, cross fire, volley of grapeshot, whiff of the grape, feu d'enfer, cut, thrust, lunge, pass, passado, carte and tierce, home thrust, coupe de bec, kick, punch, battue, razzia, Jacquerie, dragonnade, devastation, eboulement, assailant, aggressor, invader, base of operations, point of attack, echelon, attacking, aggressive, offensive, obsidional, up in arms, on the offensive, Int, up and at them!, the din of arms, the yell of savage rage, the shriek of agony, the groan of death, their fatal hands no second stroke intend, thirst for glory quells the love of life.