Medium Attachment Unit

Not to be confused with Media Access Unit.
A thicknet MAU. The vampire tap on this specimen is still attached to a piece of "frozen yellow garden hose" 10BASE5 network cable, which has been cut.
A thinnet MAU, showing a single BNC and a DA15 connector.
Two Medium Attachment Units (MAUs). The units shown are backwards compatibility-oriented 10BASET MAUs, not older 10BASE5 or 10BASE2 MAUs.
A collection of old Ethernet equipment. At the top of the image, from left to right: A thinnet MAU with passthrough BNC connectors and a DA15 connector, a thicknet MAU with passthrough N connectors and a DA15 connector, and an AUI cable for connection of a MAU to the DA15 port on a network card.
A 16-bit ISA network card engineered for compatibility with existing equipment. It includes both an AUI for connection to an external MAU (of any type) and its own MAU-type circuitry integrated on the board, which it uses in case of direct connection of thinnet cable to its BNC connector or twisted-pair cable to its 8P8C connector.

A Medium Attachment Unit (MAU) is a transceiver which converts signals on an Ethernet cable to and from Attachment Unit Interface (AUI) signals.

On original 10BASE5 (thicknet) Ethernet equipment, the MAU was typically clamped to the Ethernet wire via a vampire tap and connected by a multi-wire cable to the computer via a DA-15 port, which was also present on the network interface controller (NIC). This AUI cable could be up to 50 metres (160 ft) long, but was typically much shorter. With later standards, thicknet vampire taps and N connectors gave way to BNC connectors (for thinnet coax cables) and 8P8C connectors (for twisted-pair cables). MAUs for these were still connected to NICs via AUI cables, but soon the MAU ceased to be a separate adapter and was generally integrated into the NIC. Eventually, the entire Ethernet controller was often integrated into a single integrated circuit (chip) to reduce cost.

In most modern switched or hubbed Ethernet over twisted pair systems, neither the MAU nor the AUI interfaces exist (apart, perhaps as notional entities for the purposes of thinking about layering the interface), and the category 5 (CAT5) (or better) cable connects directly into an Ethernet socket on the host or router. For backward compatibility with equipment that still had external AUI interfaces only, adapter-type MAUs with 10BASE2 or 10BASE-T connectors long remained available after the obsolescence of original vampire tap MAUs, but even adapter-type MAUs have become very rare as of the 2020s.