(
http://www.filmsound.org/terminology/thx.htm)
THX are a design and quality control system for the playback enviroment in a theatre.
(
http://www.howstuffworks.com/thx1.htm)
THX, named after THX 1138 (George Lucas' first feature film), was developed at Lucasfilm in the early 1980s. George Lucas wanted a way to ensure a movie would look and sound exactly the same no matter where it was played. Tomlinson Holman, former corporate technical director for Lucasfilm, and a team of THX engineers took on the challenge of developing a baseline set of standards.
They noted several theater shortcomings that might negatively affect an audience's enjoyment of a film:
Outside noise (from the lobby or other auditoriums)
Inside noise (from the projector or air conditioning)
Audio distortion
Obstructed or uncomfortable viewing angles
Reverberation in the auditorium
Insufficiently bright images
Unequalized or poor audio