PDU - Protocol Data Unit

See Protocol Layers:

Let's visualize this with an example. Consider sending an email. It's going to:

1. Pass that down to TCP (highest layer) to create a TCP level 4 PDU (essentially, it's a packet at that level)
2. Then this is passed to IP (next lowest layer) to create an IP level 3 PDU
3. Then this gets passed to Ethernet (... and the process continues)

All of these are just headers and then payloads:

Really the header at the left tells you what struct to load for the next higher level header, until you've run out of levels. When a lower level considers the payload of the higher level, that's called encapsulation.