...the number 10 in Roman numerals. Or the solution to all those treasure maps in the stories we heard as children.
Those two concepts were marvelously combined in the third of the "Indiana Jones" movies along with the immortal line, "'X' never, ever marks the spot." And then it did.
Other x-related things ("related" I said, not the other thing) are x-rays, x-files, x as in "solve this algebra equation for 'x'". All extremely (sorry!) complicated.
That's enough to be going on with...I can't wait for what I do with "Z". But you'll have to.