THX
  A trademark licensed to movie theaters and manufacturers of home theater products, identifying compliance with the performance parameters set by LucasFilm Ltd. for commercial and home theater systems. THX develops standards for the playback environment regardless of film format.

Specifications for motion picture sound systems and projection are also licensed by Lucasfilm Ltd. (Various parts of the home video chain, including laserdiscs and home theater equipment, are also licensed.) The only part of the theatrical system manufactured by Lucasfilm is the speaker crossover network; other parts, such as amplifiers and speakers, must be on the "approved" THX list. Installation procedures in a THX theater also must follow rigorous Lucasfilm specifications. The name is a double entendre, partly being derived from the name of George Lucas’ first feature film, THX-1138, and partly as an acronym standing for Tomlinson Holman’s eXperiment, as he was the person responsible for the system design and philosophy.

To clear up a few misconceptions: THX has nothing to do with the recording of sound on a print, and therefore is not a competitor to Dolby Stereo or any of the digital release formats. Films do not "play in THX," and there is no such thing as a "THX film." Also, it has nothing to do with whether or not the soundtrack has been edited or mixed by the staff of Lucasfilm Ltd. and its Skywalker Sound facility.