When something is at its onset, it's at the beginning, just getting started, and it's often something that's not so pleasant. The onset of hurricane season is probably not the best time to visit the beach.

If you catch an infection right at the onset and treat it immediately, the infection may not last so long. You can also have an early onset of something, and that usually means something you'd rather not have at all, as with early-onset Alzheimer's, a disease that usually appears after age 65. If you begin showing signs of the disease at a younger age than usual, you're said to have early-onset Alzheimer's.

Definitions of onset
  1. noun
    the beginning or early stages
    “the onset of pneumonia”
    synonyms: oncoming
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    type of:
    the beginning of anything
  2. noun
    (military) an offensive against an enemy (using weapons)
    synonyms: attack, onrush, onslaught
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    ground attack
    an attack by ground troops
    an impetuous rush toward someone or something
    banzai attack, banzai charge
    a mass attack of troops without concern for casualties; originated by Japanese who accompanied it with yells of `banzai'
    diversion, diversionary attack
    an attack calculated to draw enemy defense away from the point of the principal attack
    incursion, penetration
    an attack that penetrates into enemy territory
    blitz, blitzkrieg
    a swift and violent military offensive with intensive aerial bombardment
    an attack that is intended to seize or inflict damage on or destroy an objective
    counterattack, countermove
    an attack by a defending force against an attacking enemy force in order to regain lost ground or cut off enemy advance units etc.
    bombardment, bombing
    an attack by dropping bombs
    fire, firing
    the act of firing weapons or artillery at an enemy
    an attack of machine-gun fire or cannon fire from a low flying airplane
    coup de main, surprise attack
    an attack without warning
    diversionary landing
    an amphibious diversionary attack
    a retaliatory charge
    the action of penetrating between or among
    a penetration of a barrier such as an enemy's defense
    the act of invading; the act of an army that invades for conquest or plunder
    a process in which individuals (or small groups) penetrate an area (especially the military penetration of enemy positions without detection)
    foray, maraud, raid
    a sudden short attack
    first strike
    the initial use of nuclear weapons to attack a country that also has nuclear weapons; considered feasible only when the attacker can destroy the other country's ability to retaliate
    surgical strike
    an attack (usually without prior warning) intended to deal only with a specific target
    preventive attack, preventive strike
    a strike that is carried out in order to deter expected aggression by hostile forces
    bombing run
    that part of the flight that begins with the approach to the target; includes target acquisition and ends with the release of the bombs
    area bombing, carpet bombing, saturation bombing
    an extensive and systematic bombing intended to devastate a large target
    antiaircraft fire
    firing at enemy aircraft
    barrage, barrage fire, battery, bombardment, shelling
    the heavy fire of artillery to saturate an area rather than hit a specific target
    the simultaneous firing of all the armament on one side of a warship
    burst, fusillade, salvo, volley
    rapid simultaneous discharge of firearms
    call fire
    fire delivered on a specific target in response to a request from the supported unit
    cover, covering fire
    fire that makes it difficult for the enemy to fire on your own individuals or formations
    concentrated fire, massed fire
    fire from two or more weapons directed at a single target or area (as fire by batteries of two or more warships)
    fire intended to neutralize or destroy enemy weapons
    counterpreparation fire
    intensive prearranged fire delivered when the immanence of enemy attack is discovered
    fire from two or more points so that the lines of fire cross
    destruction fire
    fire delivered for the sole purpose of destroying material objects
    direct fire
    fire delivered on a target that is visible to the person aiming it
    distributed fire
    fire dispersed so as to engage effectively an area target
    fratricide, friendly fire
    fire that injures or kills an ally
    hostile fire
    fire that injures or kills an enemy
    grazing fire
    fire approximately parallel to the ground; the center of the cone of fire does rise above 1 meter from the ground
    harassing fire
    fire designed to disturb the rest of enemy troops and to curtail movement and to lower enemy morale
    indirect fire
    fire delivered on a target that is not itself used as the point of aim for the weapons
    interdiction fire
    fire directed to an area to prevent the enemy from using that area
    neutralization fire
    fire that is delivered in order to render the target ineffective or unusable
    observed fire
    fire for which the point of impact (the burst) can be seen by an observer; fire can be adjusted on the basis of the observations
    preparation fire
    fire delivered on a target in preparation for an assault
    radar fire
    gunfire aimed a target that is being tracked by radar
    registration fire
    fire delivered to obtain accurate data for subsequent effective engagement of targets
    scheduled fire
    prearranged fire delivered at a predetermined time
    searching fire
    fire distributed in depth by successive changes in the elevation of the gun
    supporting fire
    fire delivered by supporting units to protect or assist a unit in combat
    suppressive fire
    fire on or about a weapon system to degrade its performance below what is needed to fulfill its mission objectives
    unobserved fire
    fire for which the point of impact (the bursts) cannot be observed
    artillery fire, cannon fire
    fire delivered by artillery
    (fencing) a counterattack made immediately after successfully parrying the opponents lunge
    terrorist attack
    a surprise attack involving the deliberate use of violence against civilians in the hope of attaining political or religious aims
    ambuscade, ambush, lying in wait, trap
    the act of concealing yourself and lying in wait to attack by surprise
    pre-emptive strike
    a surprise attack that is launched in order to prevent the enemy from doing it to you
    type of:
    military operation, operation
    activity by a military or naval force (as a maneuver or campaign)
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