Brax was an inhabited planet in the Gamma Quadrant. Its inhabitants called Q the "God of Lies", although Q claimed they did so affectionately. Vash visited Brax while in Q's company, though they were not particularly welcome. (DS9: "Q-Less")
This planet was only mentioned in dialogue.
The script of the episode uses the word "Braxite" in the pronunciation guide; presumably this is the name of the species who live on Brax. (Star Trek: Deep Space Nine Companion - A Series Guide and Script Library)
According to StarTrek.com, the planet might have been named after its star.