You are probably already familiar with the term ‘packet’. It is often used to generically describe data that will be passed between systems over a network. As data flows down the layers of the OSI model, the various layers encapsulate it, usually by adding header (and possibly trailer) information. At each layer where this happens, the term used to describe the data is different, and it should be properly referred to by its Protocol Data Unit (PDU) name. The reference below outlines the names used to describe Protocol Data Units at different OSI layers.
Application = Data
Presentation = Data
Session = Data
Transport = Segment
Network = Packet
Data Link = Frame
Physical = Bits
Notice that once passed to the Transport layer, data is properly referred to by its protocol data unit (PDU) name, a “segment”.
