(Is) this (seat) taken?
an inquiry made by a person in a theater, auditorium, etc., asking someone already seated whether an adjacent seat is available or already taken. Finally, Bill came to a row where there was an empty seat. Bill leaned over to the person sitting beside the empty seat and whispered, "Is this seat taken?" Fred: 'Scuse me. This taken? Alice: No. Help yourself.
See also: this