The fundamental unit of information transmitted over a network or over a digital communication link. Packets usually contain a header with control information about the packet type, source address, and destination address. They can also contain error-checking information. Packets have a logical structure based on the protocol used, but the general structure of a packet includes a header followed by a payload (data) and an optional trailer (footer). Packets can also have different sizes and structures depending on the underlying network architecture. A packet might also be called a datagram, a frame, or a cell.
From the perspective of the Open Systems Interconnection (OSI) reference model, the terms “packet” and “frame” have precise definitions. A packet is an electronic envelope containing information formed in one of the layers from layer 3 through layer 7 of the OSI model. A frame is an electronic envelope of information that includes the packet as well as other information from all seven layers of the OSI model.