Onset means the beginning or start of something, especially something significant or unwelcome. When doctors talk about the onset of a disease, they mean the moment when symptoms first appear. The onset of winter is when cold weather truly begins, marking the transition from fall's occasional chilly days to sustained cold.

The word often describes things that arrive with force or importance: the onset of a thunderstorm, the onset of adolescence, or the onset of the Industrial Revolution. You wouldn't typically use it for minor everyday starts. You wouldn't say “the onset of breakfast” or “the onset of recess.” Instead, onset marks beginnings that matter: transitions, changes, or developments that shift how things are.

At the onset of a school year, everything feels new and full of possibility. At the onset of a championship game, tension runs high. The word carries a sense that once this beginning happens, something important unfolds. Scientists study the onset of earthquakes to predict them better. Historians examine the onset of wars to understand their causes.

When you hear onset, think of a starting point that launches something bigger, something worth noting.