in ‧ trud ‧ er /ɪnˈtruːdə $ -ər/ BrE AmE noun [countable]
1 . someone who illegally enters a building or area, usually in order to steal something:
The police think the intruder got in through an unlocked window.
2 . someone who is in a place where they are not wanted:
At first I felt like an intruder in their family.