A protocol data unit at OSI layer three, the network layer, is a called a packet. A packet consists of a layer three header, such as an IP header, appended to a payload including layer four and higher headers and/or application data.
The word packet is also used in a generic way to refer to any discrete bundle of data for network transmission. So the word packet is sometimes used loosely when more precise terms such as frame or segment might also apply.
The term packet switching (in contrast to circuit switching) refers to any network transmission method that moves discrete packets independently from one another. In packet switched networks, the provisioning of a physical pathway is left to lower level protocols. Packet delivery is assured by routing the packet toward the layer three destination address found in the header encapsulating each packet.