The "X-factor" was a term used by the Confederate Marine Corps by the Guild Wars, for a talent that some marine marksman/snipers demonstrated. Specifically, it was the ability to seemingly slow the passage of time as they took their shots. Experts were hired to study the phenomena in the hope of being able to duplicate it, though as of 2488, had yet to succeed. There was a possibility raised by one psychologist that the trait was psionic in nature.
The ability was very rare, and only one in a thousand snipers/sharpshooters demonstrated it.