The ENCOM 511 is the name of the supercomputer that is home to the Master Control Program. While the actual computer and the name, "ENCOM 511" don't actually appear in the movie TRON, it was mentioned in one of the many trailers for the movie. Over an ambient synthesizer sound, the narrator in the trailer introduces the 511 as the camera moves towards the 511.
|“||The computer: an extension of the human intellect. The ENCOM 511, center of the most calculating intelligence on earth. Programmed by Master Control to survive... by all means. Soon, the ultimate tool will become the ultimate enemy.||”|
—Narrator for Tron Trailer
Even though the 511 didn't appear in the movie, it is probably considered canon to the TRON Universe and has been accepted by many TRON fans to be canon.