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Noun: onset  'ón,set
  1. The beginning or early stages
    "the onset of pneumonia";
    - oncoming
  2. (military) an offensive against an enemy (using weapons)
    "the onset began at dawn";
    - attack, onslaught, onrush

Derived forms: onsets

Type of: military operation, op [informal], operation, start

Encyclopedia: Onset, Massachusetts