Stands for "Sonic Adventure: DX" or "Sonic Adventure: Directer's Cut". SADX is the directer's cut of the old "Sonic Adventure" on the "Sega Dreamcast" system. However SADX brings better looking graphics than that of the Dreamcast, including improved character models, textures, and world textures that fit the current area better. The original Sonic Adventure was released on "December 23, 1998". SADX was released 2 years after "Sonic Adventure 2" and "Sonic Adventure 2 Battle" were released, in 2003, on the "Nintendo Gamecube" and PC systems. In a nutshell, SADX is a remake of the original Sonic Adventure on the Dreamcast.
dude 1: hey dood wanna watch me play some SADX?
dude 2: Some Sonic Adventure: Directer's Cut? Yea!
dude 1: you coulda just said SADX and saved 2 seconds of your life.