What Does The 'X' In X-Men Stand For? (It Might Not Be What You Think)
Because Professor Charles Xavier has been leading the team since their 1963 comic book debut, it makes sense that the X-Men would derive their name from their telepathic chief. But that isn't really the case. In fact, the very first "X-Men" comic — written by Stan Lee, drawn by Jack Kirby, inked by Paul Reinman, and lettered by Sam Rosen — explains exactly why Xavier's team of mutants got that title, and it has nothing to do with his name. At this point, Xavier's School for Gifted Youngsters consists of just Cyclops, Iceman, Beast, and Angel, so when a young Jean Grey is telepathically summoned to the school by Xavier, she has some questions.
He explains to her, "You, Miss Grey, like the other four students at this most extraordinary school, are a mutant! You possess an extra power ... one which ordinary humans do not!! That is why I call my students... X-Men, for EX-tra power!" So while the obvious conclusion would be that he named the team after himself, a guy named Professor X leading the X-Men is more or less a coincidence. Despite that, the letter X will continue to play a pivotal role in the group's lore.
Weapon X and beyond
In addition to Charles Xavier heading up the team, it just so happens that the mutant gene shared by all X-Men is the X-Gene, which is the source of their various powers. On top of that, the government's secret Weapon X program has birthed the likes of Wolverine and Deadpool. The X-Cutioner is a Marvel villain who hunts mutants. The list goes on and on.
The reasoning behind the X-Men's title has changed over the decades. In the 2000 movie, for example, Patrick Stewart's Professor X says that his students chose the name themselves. But while the inspiration for the name can shift from writer to writer, its true origin was laid out in 1963. The popularity of the "X-Men" series is likely what retroactively led to the letter "X" popping up all across the Marvel Comics universe. Things like the Weapon X Program — and, by extension, Wolverine — arguably wouldn't exist without the team's title. And it all was because Professor X was having some fun with wordplay.