802.1X is a port-based network access control (PNAC) protocol that provides an authentication mechanism to devices wishing to connect to a LAN or WLAN. It is widely used for secure network access, ensuring that only authenticated devices can access the network, whether they are connecting via wired or wireless means.
802.1X works in conjunction with an authentication server, such as RADIUS, to validate the credentials of devices trying to connect.References: CompTIA Network+ study materials.