There are no Magic: The Gathering sets that begin with X (or Y, for that matter) and I failed to find any supplementary products as well. My original plan was to talk about Tenth Edition, since its set symbol is "X". I wasn't sure how to do the title of the post, though. "X Set Symbol" wasn't exactly rolling off the tongue. So I decided to go with this. It doesn't hurt that it makes for a short and sweet entry!
The "X" in a casting cost of a spell is a variable mana cost usually determined by the casting player. It almost always correlates to a "X "variable in the card's text box. "X" costs may exist in abilities of other cards as well.
From the MTG Comprehensive Rules about "X": "Many objects use the letter X as a placeholder for a number that needs to be determined. Some objects have abilities that define the value of X; the rest let their controller choose the value of X."
So some times "X" will already have predetermined parameters to calculate its value, otherwise the controller of that spell or ability chooses and announces the value of "X" when casting said spell or activating said ability.
If a card with a "X" casting cost is anywhere but on the stack, its "X" is always 0.
There are a few other nuances, but those are the major points!
Do you have any favorite cards that have a "X" in its casting cost? What about cards with a "X" in its ability casting cost? Can you think of any Magic: The Gathering product beginning with "X"?