Dolby Stereo, is Dolby Laboratories' trademark for its various analog stereo cinema sound formats.
Two basic systems used this name: the Dolby SVA (stereo variable-area) system used with optical soundtracks on 35mm film, and Dolby Stereo 70mm, which referred to Dolby noise reduction on 6-channel magnetic soundtracks on 70mm prints.
Read more about Dolby Stereo: Dolby SVA, History, The Dolby Stereo Matrix, Dolby Stereo 70 Mm Six Track, Ultra Stereo, Dolby Surround, Dolby Pro Logic