n. (also x) (pl. Xs or X's)
1 the twenty-fourth letter of the alphabet.
2 (as a Roman numeral) ten.
3 (usu. x) Algebra the first unknown quantity.
4 Geom. the first coordinate.
5 an unknown or unspecified number or person etc.
6 a cross-shaped symbol esp. used to indicate position (X marks the spot) or incorrectness or to symbolize a kiss or a vote, or as the signature of a person who cannot write.
symb. (of films) classified as suitable for adults only.
Formerly used in the UK to indicate that persons under 18 would not be admitted; it was replaced in 1983 by 18, but is still used in the US.
Useful english dictionary. 2012.