There are different Usages of the letter X.
1. the twenty-fourth letter of the English alphabet
A B C D E F G H I J K L M N O P Q R S T U V W X Y Z
This letter denotes the next after W in a set of items, categories, sizes, etc. e.g. It also denotes the twenty-fourth of twenty-four or more hypothetical people or things.
X = Adult Movie Rating. X = Cross = Incorrect Answer. X = Sign in place of Signature for a person who can not write. Besides, X letter of the English Alphabet is used in Mexico like MEX as the International Vehicle Registration.
X = Independent Mathematical Variable. X = as in x-axis = horizontal axis.
The most Common Noun which is used to learn this letter at the Nursery level is X-mas; X for X-mas = Christmas.