Meaning of X in English

X

1. 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. 2.

symb. (of films) classified as suitable for adults only.

Usage:

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.

Oxford English vocab.      Оксфордский английский словарь.