What does thx stand for?

The third act is an extended escape sequence, featuring a futuristic and often-copied car-chase sequence through a tunnel network. THX and SRT steal two cars, but the latter immediately crashes. It is uncertain whether SRT survives the impact, although the film's script indicates he does not. THX flees to the limits of the city's underground road network while being chased by android police, and eventually locates an access tunnel that leads up to the surface. The police pursue THX up the ladder, but the expense of his capture exceeds its budget, compelling the police to retreat, leaving THX to climb out and stand, for the first time, in the harsh surface sunlight. As THX stands before the setting sun, birds intermittently fly overhead, indicating that life is possible on the surface.