X is for lots of things

It turns out X is for quite a lot of things:

In mathematics, ‘x’ is commonly used as the name for an independent variable or unknown value.
‘x’ is used to signify the multiplication operation when a more appropriate glyph is unavailable.
As a result of its use in algebra, X is often used to represent unknowns in other circumstances (e.g. Malcolm X).
In the Cartesian coordinate system x is used to refer to the horizontal axis.
X-rays are so called because their discoverer did not know what they were.
X has been used as a namesake for a generation of humans: Generation X, commonly abbreviated to Gen X. It is the generation born after the baby boom ended, ranging from 1961 to 1981.
An X-shaped mark has traditionally been used by the illiterate in lieu of a signature, and is also used to indicate a signature line on forms.
X marks are used to indicate the concept of negation or incorrect, the opposite of a Tick (check mark). They are also used as a substitute for the check mark (most notably on election Ballot papers)
X is commonly used as a generic mark (selecting an item on a form, indicating a location on a map, etc.).
The common custom of placing Xs on envelopes, notes and at the bottom of letters to mean kisses dates back to the Middle Ages, when a Christian cross was drawn on documents or letters to mean sincerity, faith, and honesty.
Usually in art or fashion, the use of X indicates a collaboration with two or more artists.
In cartoons, a dead character’s eyes are often drawn as Xs.
In mapping by the standards of the USGS, an x-type mark is used to denote the point referred to by an elevation marking on topographical maps.
Maps leading to hidden treasure often denote the treasure with an X. The expression “X marks the spot” is related to these treasure maps

That’s quite a lot of meanings attached to two little crossed lines! And thanks Wikipedia editors for your X article :)