Full-duplex, or simply "duplex," is a type of communication in which data can flow two ways at the same time. Full duplex devices, therefore, can communicate back and forth simultaneously.
Telephones are common examples of full-duplex devices. They allow both people to hear each other at the same time. In the computer world, most network protocols are duplex, enabling hardware devices to send data back and forth simultaneously. For example, two computers connected via an Ethernet cable can send and receive data at the same time. Wireless networks also support full-duplex communication. Additionally, modern I/O standards, such as USB and Thunderbolt, are full-duplex.
The terms duplex and full-duplex can be used interchangeably since both refer to simultaneous bidirectional communication. Full-duplex is often used in contrast to half-duplex, which refers to bidirectional communication, but not at the same time. Simplex communication is even more limited and only supports data transmission in one direction.
NOTE: Full-duplex is sometimes abbreviated "FDX."