Wikipedia
PCI-X
PCI-X, short for Peripheral Component Interconnect eXtended, is a computer bus and expansion card standard that enhances the 32-bit PCI local bus for higher bandwidth demanded mostly by servers and workstations. It is a double-wide version of PCI, running at up to four times the clock speed, but is otherwise similar in electrical implementation and uses the same protocol.
It has been replaced in modern designs by the similar-sounding PCI Express (officially abbreviated as PCIe), with a completely different connector and a very different logical design, being a single narrow but fast serial connection instead of a number of slower connections in parallel.