Longman Dictionary of Contemporary English

~1 plural X's,x's ~ n 1 the 24th letter of the English alphabet 2 the number 10 in the system of roman numerals 3 technical a letter used in mathematics to represent an unknown quantity or value  (if 3x=6, x=2) 4 a mark used to show that a written answer is wrong 5 a mark used to show that you have chosen something on an official piece of paper, for example when voting 6 a mark used instead of a signature by someone who cannot write 7 a mark used to show a kiss, especially at the end of a letter 8 a letter used to show that a film is not suitable for people under 18 9 a letter used instead of someone or something's real name because you want to keep it secret or you do not know it  (At the trial, Ms X said that she had known the defendant for three years.) 10 X number of used to say that there are a certain number of people or things when the exact number is not important 11 X marks the spot used on maps in adventure stories to show that something is buried in a particular place ~2 v X out sth phr v AmE to mark or remove a mistake in a piece of writing using an X cross out cross1