X

X.21
A common standard for synchronous data connections using balance voltage-differential signals over a cable.

X.25
A mature protocol, defined by the ITU and used in satellite systems and public data networks, to provide reliable data communications over packet switched networks.

XPD
Cross Polar Discrimination. A measure of how effective a terminal is at distinguishing between signals in opposite polarisations. This is important for both transmit and receive and can sometimes be optimised by adjusting the feed system

XPOL
signals that are linearly and horizontally polarised