An intruder is someone who enters a place where they don't belong and aren't welcome. When a stranger climbs through a window into someone's home at night, they're an intruder. When an unauthorized person slips into a restricted area of a building, they're an intruder.

The word carries a sense of violation and unwanted presence. An intruder has crossed a clear boundary, entering space that belongs to others without permission. A trespasser on private property is an intruder. Someone who sneaks into a school after hours is an intruder.

You might also hear intruder used more broadly. If a raccoon breaks into your garage looking for food, you could call it an intruder. In some contexts, people describe unwanted thoughts that pop into their minds as intrusive, from the same root.

Security systems often have intruder alarms designed to detect unauthorized entry. The word reminds us that certain boundaries matter: some spaces are private, protected, or restricted for good reasons, and crossing those lines without permission is serious.