x1

'by 1' - often used to classify the bandwidth of a link in units of used lanes, here one lane

x4

'by 4' - often used to classify the bandwidth of a link in units of used lanes, here four lanes

x8

'by 8' - often used to classify the bandwidth of a link in units of used lanes, here eight lanes

XAUI

acronym for 10GbE

XLINK

set of adaptor cards to connect legacy PCI or cPCI systems to a MicroTCA system using PCI/PCIe