Onset

Onset generally refers to the start of something negative, unwelcome, or intense, often an event that seems to happen suddenly or that you did not choose.

• Meaning: The beginning or initial stage of something, especially something bad.
• Connotation: Negative, unwelcome, or inevitable.
• Typical Usage: Often used with diseases, weather, or negative conditions:
  • The onset of flu symptoms.
  • The onset of winter.
  • The onset of war.

Outset

Outset simply refers to the start or beginning of an activity, process, or endeavor and carries a neutral or positive connotation. It is often used to refer to a starting point of an undertaking or a journey.

• Meaning: The beginning or start of a process, event, or period.
• Connotation: Neutral, often referring to a planned or chosen start.
• Typical Usage: Almost always used in the phrase “at the outset” or “from the outset”:
  • He made his intentions clear at the outset of the meeting.
  • There were problems with the project from the outset.
  • She felt positive at the outset of her new career.