In number theory, Fermat's Last Theorem (sometimes called Fermat's conjecture, especially in older texts) states that no three positive integers a, b, and c satisfy the equation an + bn = cn for any integer value of n greater than 2. The cases n = 1 and n = 2 have been known to have infinitely many solutions since antiquity.This theorem was first conjectured by Pierre de Fermat in 1637 in the margin of a copy of Arithmetica where he claimed he had a proof that was too large to fit in the margin.

The New York City Subway currently uses various letters and numbers to designate the routes that trains use over the differing lines in the system.

Xnet (former eXgae) is a non-profit activist platform that develops and promotes alternative models for cultural dissemination and royalty management and work in different fields related to digital rights, networked democracy and freedom of expression.

XNU is the computer operating system kernel developed at Apple Inc. since December 1996 for use in the macOS operating system and released as free and open-source software as part of the Darwin operating system.

X&Y (stylized as X & Y) is the third studio album by the British rock band Coldplay. It was released on 6 June 2005 by Parlophone in the United Kingdom, and a day later by Capitol Records in the United States.

XYY syndrome is a genetic condition in which a male has an extra Y chromosome. Symptoms are usually few.

The XNOR gate (often written Exclusive-NOR, sometimes: EXNOR, ENOR, NXOR, XAND) is a digital logic gate whose function is the logical complement of the exclusive OR (XOR) gate.

Xylem is one of the two types of transport tissue in vascular plants, phloem being the other. The basic function of xylem is to transport water from roots to shoots and leaves, but it also transports nutrients.

The xylophone (from the Greek words ξύλον—xylon, "wood" + φωνή—phōnē, "sound, voice", meaning "wooden sound") is a musical instrument in the percussion family that consists of wooden bars struck by mallets.