(PCI eXtended) An enhanced PCI bus technology originally developed by IBM, HP and Compaq that is backward compatible with existing PCI cards. PCI and 32-bit PCI-X slots are physically the same, and PCI cards can plug into PCI-X slots. PCI-X cards will run in PCI slots, but at the slower PCI rates. The 64-bit PCI-X slots are longer.

First introduced in 1999, PCI-X offered increased speed over PCI and has steadily increased to more than 30 times that of the original PCI bus. Do not confuse PCI-X with PCX, which is the abbreviation of PCI Express. For a comparison of all PCI technologies, see PCI-SIG.