The entirety of a packet at one Layer becoming the payload section at another is know as?

It is Encapsulation

This is inclusion of one data structure within another structure so as that another structure is hidden for time being. An example a TCP/IP formatted data packet can be encapsulated within an ATM frame (another kind of transmitted data unit) within the context of transmitting and receiving the ATM frame, the encapsulated packet is simply a stream of bits between the ATM data that describes the transfer.

Its known as encapsulation.