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 :)