intruder
n. countable B2 Upper Intermediate US //ˌɪnˈtɹudɝ// UK //ɪntɹˈuːdɐ// in·trud·er
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.
From intrude + -er.
Usage
Often used in legal or security contexts to distinguish between a guest and an unauthorised person.