Hello Harshit
PDU is short for Protocol Data Unit. It is a generic term used to refer to a piece of data at various layers of the OSI model. For example, at the Transport Layer, if we’re using TCP, the PDU is called a Segment. If we’re using UDP, it is called a Datagram. At the Network Layer, it is called a Packet. At the Datalink Layer, it is called a Frame. We can use the term PDU to refer to these units in general. Here’s an example of the OSI model that includes the names of the PDUs at the various layers.
I hope this has been helpful!
Laz