The horizontal axis by convention. Also the mark for an unknown. A named placeholder rather than a thing.
The letter descends from Greek Χ (chi), by way of the Latin alphabet's X. Its use as the algebraic mark for an unknown quantity is credited to René Descartes, who in La Géométrie (1637) fixed the convention of naming unknowns with the alphabet's last letters (x, y, z) and knowns with its first (a, b, c) — a convention later mathematicians kept rather than one forced by the letter's own history.
One letter doing two jobs: naming a position along the horizontal, and, older still, standing in for whatever is not yet known. A named gap stays a gap — naming it only makes it easier to point at.