802.1X
A standard requiring devices to authenticate before they are granted access to a wired or wireless network.
Rather than a shared passphrase, each device or user authenticates individually against a directory, usually through RADIUS. Access can then be granted per identity, including automatic VLAN assignment.
It removes the problem of a shared wireless key that everyone knows and nobody can change, and it means a device plugged into a wall port does not automatically get onto the network.
Where this comes up
802.1X sits inside our networks and infrastructure work. If you are trying to work out what this means for your own environment rather than in the abstract, that is the page worth reading next, and a short call will get you a straight answer faster than either.