An unknown quantity or unknown value.

A placeholder for an unknown, suppressed or hypothetical name.

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

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. / (lacrosse) The spot behind the goal.

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

(slang) Ecstasy, a particular street drug.

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

(informal) Christ

(Canada, US) Cross, crossing.

(informal) Abbreviation of extreme.