X Definition

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

(fandom slang) Used between the names of two characters to denote a ship, particularly in anime, manga, and video games.

(slang) Used to replace a -ks- sound, as in hax (hacks), punx (punks), pix (pics).

(computing) Any of a family of instruction set architectures based on the Intel 8086 microprocessor.

An ITU-T Recommendation (1988) for a public key infrastructure (PKI). X.509 establishes a hierarchical structure of certificate authorities (CAs) that issue digital certificates, which are electronic credentials that authenticate the identity of users on the Internet and intranets. See also authentication, CA, digital certificate, Internet, intranet, and PKI.

An international standard (1988) developed jointly by the International Organization for Standardization (ISO) and the ITU-T to support the e-mail name and address lookup requirements of X.400. X.500 provides for global directory services that theoretically enable network managers to store information about all users, machines, and applications in a distributed fashion. X.500 is a very robust global directory standard that requires significant computational resources to implement and, therefore, is criticized for being over-engineered. See also e-mail, ISO, ITU-T, LDAP, X.400, and X Series.

The symbol for reactance. See reactance.

The ITU-T Recommendation for access to an X.25 packet network in synchronous dial-up mode through a public switched telephone network (PSTN), an integrated services digital network (ISDN), or a circuit-switched public data network (PDN). See also dial-up access, PDN, PSTN, synchronous, and X.25.

In mathematics, the symbol for an unknown quantity, person, factor, or thing. X is the first in a set of two or three unknowns comprising x, y, and z. In telecommunications, for example, xDSL refers to any of a group of Digital Subscriber Line services. See also x-axis, xDSL, y, and z.