The AUI (attachment unit interface) is the 15-pin physical connector interface between a computer's network interface card (NIC) and an Ethernet cable. On 10Base-5 ("thicknet") Ethernet, a short cable is used to connect the AUI on the computer with a transceiver on the main cable. In 10Base-2 or "thinnet" Ethernet networks, the NIC connects directly to the Ethernet coaxial cable at the back of the computer.
IEEE 802.3, the Ethernet standard, defines the AUI 15-pin physical layer interface. This interface is also called a DB-15 interface or a DIX interface (DIX refers to the three major companies who helped standardize Ethernet: Digital Equipment Corporation, Intel, and Xerox).