A person whom one does not know; a person who is neither a friend nor an acquaintance.
One who has not been seen for a long time.
A newcomer.
An outsider or foreigner.
One not admitted to communion or fellowship.
One not privy or party to an act, contract, or title; a mere intruder or intermeddler; one who interferes without right.
A person or thing whose nature or value is a mystery.
Used other than figuratively or idiomatically: see unknown, quantity.