Stranger is a person whom one does not know or with whom one is not familiar.
In modern usage, stranger can also refer to:
1. A newcomer to a particular place ("She's a stranger to these parts")
2. Someone unacquainted with or unaccustomed to something ("He's no stranger to hard work")
3. In internet slang, an anonymous person online ("Don't talk to strangers in chat rooms")
4. In the phrase "stranger things", meaning unusual or unexpected occurrences