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The twenty-fourth letter of the modern English alphabet and the twenty-first of the ancient Roman one, in the latter an adoption of Greek khi.

X is traditionally used to signify an unknown person or thing, or to note the location of a particular place, as in X marks the spot.

X chromosome in humans and other mammals, a sex chromosome, two of which are normally present in female cells (designated XX) and only one in male cells (designated XY).

X files a cult American television series (1993– , created by Chris Carter) in which two special agents, Fox Mulder and the more sceptical Dana Scully, repeatedly investigate cases which appear to involve the paranormal; final proof of extra-terrestrial activity, however, is always lacking, although it is indicated that this may be deliberately suppressed by government agency. The slogan of the series, ‘The truth is out there’, has become a catchphrase.

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