X.
An unknown quantity or unknown value.
Meaning & usage
An unknown quantity or unknown value.
Let X represent the forecast traffic flow in 20 years’ time.
Where this word comes from
Introduced by René Descartes in the context of mathematical notation, but has since widened to other contexts.
A placeholder for an unknown, suppressed or hypothetical name.
The woman known until now as Witness X has been unmasked after a court ruling.
Suppose that Mr and Mrs X have been married for many years.
A surname, used by those who have had their identity or heritage, including their proper ancestral names, erased or forgotten.
Malcolm X
An online social media network, formerly known as Twitter.
Where this word comes from
Introduced by René Descartes in the context of mathematical notation, but has since widened to other contexts.
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.
The spot behind the goal.
Intersex or non-binary (in passports and identification documents).
Ecstasy, a particular street drug.
Where this word comes from
Presumably by abbreviation of the pronunciation of ecstasy.
Suitable only for those aged sixteen or (later) eighteen years and over.
Obscene; adult; pornographic; XXX.
Christ
Where this word comes from
From Christ by abbreviation, orthographic borrowing from Ancient Greek Χ (Kh, letter chi), from Χριστός (Khristós, “Christ”).
Cross, crossing.
Kennington via Charing X
Where this word comes from
From cross, due to the X symbol being a cross saltire.
The X Window System, a windowing system for bitmap displays commonly used in Unix-like systems.
Where this word comes from
Named after its predecessor, the W Window System; X follows W in the Latin alphabet.
Alternative spelling of ex (“to delete; to cross out”).
Abbreviation of extreme.
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Definitions, grammatical labels, IPA, etymology and examples: Wiktionary contributors, extracted by Kaikki / Wiktextract. Adapted under CC BY-SA 4.0. Selected fields and authored examples are shown; third-party quotations and audio are omitted. See contributor history.
Last enriched 2026-09-10. Definitions are in English. Romanizations are shown when supplied by the source; syllable estimates are omitted for unsupported scripts.