What is the process of taking data from the layer above adding a header with addressing information in it called?

The process of taking data from the layer above and adding a header with addressing information is called "encapsulation." In networking, this occurs as data is passed down through the OSI model layers, where each layer adds its own header (or trailer) to the data unit, creating a new protocol data unit (PDU) for that layer. This encapsulation ensures that the data is properly routed and processed by the appropriate protocols at each layer.