Protocol Data Unit (PDU)

A protocol data unit or PDU is the generic name used to refer to the entity created at each layer of the OSI Model or the TCP-IP model. The specific name of each entity at each layer is as follows:

• Datalink layer: Ethernet frame
• Network layer: IPv4 or IPv6 packet
• Transport layer: TCP segment or UDP datagram depending upon the protocol used.

At the physical layer we typically use the terms "bits" or "words" or even voltage levels (for copper cabling) or light pulses (for fiber optics). The terms used also depend upon the type of encoding that takes place.

Links:

https://forum.networklessons.com/t/how-does-a-switch-learn-mac-addresses/865/113?u=lagapides

https://networklessons.com/cisco/ccna-routing-switching-icnd1-100-105/introduction-to-the-osi-model