x

UK: 'X', 'x': /ˈɛks/ US: 'X', 'x': /ɛks/ ,'X', 'x': (eks)

Collins Concise English Dictionary © HarperCollins Publishers::

x, X / ɛks/ noun (x's, X's, Xs)

1. the 24th letter and 19th consonant of the modern English alphabet
2. a speech sound sequence represented by this letter, in English pronounced as ks or gz or, in initial position, z, as in xylophone

x symbol

1. ex
2. the x-axis or a coordinate measured along the x-axis in a Cartesian coordinate system
3. an algebraic variable

WordReference Random House Learner's Dictionary of American English © 2026

ˈRo•man ˈnu•mer•al,  n. [countable]

1. any of the numerals in the ancient Roman system of notation, still used occasionally. The basic symbols are I (=1), V (=5), X (=10), L (=50), C (=100), D (=500), and M (=1000).

WordReference Random House Unabridged Dictionary of American English © 2026

Ro′man nu′merals, 

1. the numerals in the ancient Roman system of notation, still used for certain limited purposes, as in some pagination, dates on buildings, etc. The common basic symbols are I (=1), V (=5), X (=10), L (=50), C (=100), D (=500), and M (=1000). The Roman numerals for one to nine are: I, II, III, IV, V, VI, VII, VIII, IX. A bar over a letter multiplies it by 1000;
   thus, X̄ equals 10,000. Integers are written according to these two rules: If a letter is immediately followed by one of equal or lesser value, the two values are added;
   thus, XX equals 20, XV equals 15, VI equals 6. If a letter is immediately followed by one of greater value, the first is subtracted from the second;
   thus, IV equals 4, XL equals 40, CM equals 900. Examples: XLVII(=47), CXVI(=116), MCXX(=1120), MCMXIV(=1914). Roman numerals may be written in lowercase letters, though they appear more commonly in capitals.

• 1725–35

Collins Concise English Dictionary © HarperCollins Publishers::

X symbol

1. (formerly, in Britain) indicating a film that may not be publicly shown to anyone under 18. Since 1982 replaced by symbol 18
2. (as modifier): an X film
3. denoting any unknown, unspecified, or variable factor, number, person, or thing
4. (on letters, cards, etc) denoting a kiss
5. (on ballot papers, etc) indicating choice
6. (on examination papers, etc) indicating error
7. for Christ; Christian
8. (Roman numeral) ten
   See Roman numerals

Etymology: (sense 6) from the form of the Greek letter khi (Χ), first letter of Khristos Christ