Designation for a band of humans under the command of the Goa'uld Apophis, training for infiltration of Stargate Command on Earth. SG-X was the marking of the fictitious unit as they practiced battle maneuvers against a band of Jaffa. The team's leader was "Captain" Kyle Rogers.
SG-X trained at Apophis's training camp for at least a year, and continued to train even after Apophis's death, which they had not realized occurred.
Rules of Engagement - SG-1 comes upon a training camp apparently occupied by Jaffa warriors and humans training under the guise of SG-X.