In networking, any part of the network whose data transmission paths are unpredictable and vary from session to session.
Clouds are often used in networking diagrams to represent packet-switching services. In these services, a packet sent from one node to another follows an unpredictable path, since at any given moment different routers or other devices can be used to forward the packet toward its destination.
The Internet is an example of a packet-switching cloud for TCP/IP networking, since data sent between two points can travel over many possible paths. This is why the Internet is graphically represented as a cloud in drawings of wide area networks (WANs). Other examples of packet-switching services include frame relay and X.25 networks.
Circuit-switched services are often represented as clouds as well. In circuit-switched services, communication switches at various telco and carrier central offices (COs) and switching facilities are temporarily used for establishing circuits between two communicating nodes. Each time communication is terminated and reestablished, different sets of switches can be used.