What is the Dolby Pro Logic audio format?

Dolby Pro Logic is a surround sound processing technology designed to decode soundtracks encoded with Dolby Surround. Dolby Surround was originally developed by Dolby Laboratories in 1976 for analog cinema sound systems as Dolby Analog SR (Spectral Recording). The Analog SR format was adapted for home use in 1986 as Dolby Surround which was then replaced by the newer and improved Pro Logic system. However, the term "Dolby Surround" is still used to describe the encoding technology or matrix-encoded soundtrack, whereas Pro Logic refers to the decoding technology/processor.