A windowed envelope is a conventional envelope with a plastic (typically film or BOPS Bi-oriented polystyrene films) window to allow the recipient's address to be printed on the paper contained within..
Used widely in the postal world to send a variety of correspondence such as magazines, brochures, catalogs and newspapers, plastic envelopes are an alternate option to shipping these items in rigid cartons or cardboard boxes, and are also known as poly mailers or poly mailer bags.