The word "intruder" refers to someone or something that enters a place without permission or invitation. It typically implies that the person or object is unwanted or unauthorized. Full definition
They consider other animals as intruders in their territory. (austindog.org)
When proteins are not properly broken down during digestion, they are identified as intruders instead of nutrients and the enterocytes attack them. (wagwalking.com)
The effectiveness of intruder alarm systems depends on the reliability of the Alarm Receiving Centre monitoring activation signals, both to determine genuine activations and, more importantly, to ensure a quick response. (educationbusinessuk.net)