intruder
nounUK /ɪnˈtruː.dər/
US /ɪnˈtruː.dɚ/
A person who goes into a place where they are not allowed.
The alarm went off when an intruder climbed through the window.
OTHER MEANINGS
2. Someone who joins a group or activity without being asked or wanted.
The boys felt like intruders at the party where they knew no one.
3. An animal or plant that moves into a place where it does not usually live.
The new plant spread quickly and became an intruder in the garden.
4. A person who interrupts a private conversation or meeting.
She knocked softly, not wanting to be an intruder on their talk.