1. a person who enters a place illegally or without permission
2. someone who interrupts or interferes in a situation where they are not wanted

Examples

• The security alarm went off when an intruder broke into the house.
• Police arrested the intruder who had been hiding in the basement.
• She felt like an intruder at the private family gathering.
• The intruder was caught on camera climbing over the fence.
• He didn’t want to be an intruder in their conversation.
• Motion sensors detected an intruder in the building after hours.
• The dog barked loudly at the intruder in the yard.