According to Webster’s 20th Century Unabridged Dictionary:

• X is the 24th letter of the English Alphabet • A mark shaped like X is used to represent the signature of a person who cannot write • To indicate a particular point on a map, diagram, etc. • As a symbol for a kiss in letters, etc. • A person or thing unknown or unrevealed • The Roman numeral 10 • Christ: used also in combination as in Xmas • In chemistry, the symbol for Xenon • In mathematics, an unknown quantity • A sign of multiplication • An abscissa