what does the "x" in x-men stand for?

All the previous answers are wrong. The "X" in X-Men stands for extra. "He dubbed his students the "X-Men," because each possessed an "extra" ability that normal humans lacked (marvel.com).

It stand for their extra power as given by the X gene.

X stand for unknown. The unknown genetic factor that mutates the super heroes.

If you saw the last x-men movie they explained it. Professor Xavier was called professor X for short. After professor Xavier dies they call them selves the x-men in honor of their mentor.

I agree wthis is what I remembered.

"It stand for their extra power as given by the X gene"

Xanthan gum

it's never really been defined - in the early comics, magneto referred to them as the "x-generation" that belonged to a different species of human although they were born of natural parents, so i guess that could explain it.

Hi. Pronounced the same as "ex".

It is an X factor, which causes this mutation

Let me guess lol it got three letters it can only mean one thing ☺

(wink)