An intrusion is an unwelcome entry into a place or situation where you don't belong or weren't invited. When someone barges into your room without knocking, that's an intrusion on your privacy. When a stranger shows up at a family dinner uninvited, their presence feels like an intrusion.

The word carries a sense of violation or disruption. It means being somewhere you shouldn't be, disturbing what was happening before. A loud noise can be an intrusion on a quiet afternoon of reading. A rude question about someone's personal life can feel like an intrusion, crossing boundaries that should have been respected.

In geology, an intrusion happens when molten rock forces its way into cracks in existing rock underground, then cools and hardens. This geological meaning captures the word's essence: something pushing in where it doesn't naturally belong.

The related word intrusive describes someone or something that intrudes. An intrusive neighbor might constantly ask personal questions or drop by without warning. Intrusive thoughts are unwanted ideas that pop into your mind when you're trying to focus on something else.