A feature of home theatre audio equipment in which there are at least four speakers spaced at more-or-less even intervals around the listener. This can make movies like Aliens sound much cooler because you hear the aliens creeping up behind you. Also, in Star Wars, you hear starfighters going behind you. It heightens the experience of the movie. There are several different types of surround sound encoding: Dolby Pro Logic, Dolby Digital aka AC3, and THX. These at least are the most recognized ones.