Definition of "X"

An unknown quantity or unknown value.  examples

A placeholder for an unknown, suppressed or hypothetical name.  examples

(anthroponymy, politics) A surname, used by those who have had their identity or heritage, including their proper ancestral names, erased or forgotten.  examples

The twenty-fourth letter of the English alphabet, called ex and written in the Latin script.  examples

Any mark that looks like that letter, such as a mark made by a person who cannot read or write in lieu of a signature.  examples

(lacrosse) The spot behind the goal.  examples

Intersex or non-binary (in passports and identification documents).  examples

(slang) Ecstasy, a particular street drug.  quotations

(Britain film certificate, dated) Suitable only for those aged 16 or (later) 18 years and over.  examples

(movie rating) Obscene.  quotations  examples

(informal) Christ  examples

(Canada, US) Cross, crossing.  examples

(informal) Abbreviation of extreme.  examples