X Definition

X¹
x
noun Xs, X's, x's

1. The twenty-fourth letter of the English alphabet.
2. Anything shaped like an X.
3. An unknown or unnamed person.

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X²
symbol

1. math.
An unknown quantity; the first of a pair or group of unknown quantities. See also Y³, Z³.
Form: x (usually)
2. The Roman numeral for 10.
3. A film classified as suitable for people over the age of 17 (in the USA) or 18 (in the UK; now replaced by ‘18').
4a. A mark used to symbolize a kiss;
4b. A mark used to indicate an error;
4c. A mark used as the signature of an illiterate person, etc.
5. Christ, as in Xmas.

Etymology: from X = chi, letter of the Greek alphabet, and the first letter of Christos, the Greek form of Christ.