Stranger was the term used for anyone who was not a member of either the House of Commons or the House of Lords. It was replaced by 'Members of the Public' in November 2004. "I Spy Strangers" was the traditional request for members of the public to leave the galleries of the House of Commons so that the Chamber could sit in private. Following a request from the Modernisation Committee in 1998, the procedure has been abolished and replaced by a simpler one based on the motion 'that the House sit in private.'
Further information on this subject can be found from the following link.
Traditions and Customs of the House