intruder
n.a person who enters a place without permission, often to steal something or cause trouble.
n.a person who enters a building, grounds, or private property without invitation or legal right.
“The security alarm went off when the intruder broke a window.”
“The homeowner called the police after seeing an unknown intruder on the backyard security camera.”
“While the motion sensors are designed to detect a human intruder, they are frequently triggered by local wildlife or swaying tree branches.”
Often used in legal or security contexts to distinguish between a guest and an unauthorised person.
“Munroe, 43 at the time, then wrestled the gun away from one of the intruders and shot him.”
From intrude + -er.
Etymology adapted from Wiktionary, available under CC BY-SA 4.0.