Intruder (noun)

1. A person who enters a place or situation without permission or invitation.
2. A person or thing that interrupts or interferes with something else.

Origin:

Late middle english (as a verb): from old french entredre, based on latin intrare "to enter".

Examples:

1. The intruder was caught by the police.
2. The loud noise was an intruder in the otherwise peaceful night.
3. He was just an intruder in the conversation.
4. The new law is an intruder on our rights.
5. The intruder managed to get into the house without being detected.