The packet forwarding service of a router or switch.
Functions on a network device are divided into control plane and data plane functions, also called forwarding plane functions. Control functions are performed relatively infrequently such as system configuration and management and also updates to the forwarding table in the data plane. Data plane / forwarding plane functions are performed on every packet. The data path is the packet forwarding heart of the data plane. This means reading packet headers (parse/open and encapsulate/close), queuing packets into the forwarding table, which maps network addresses to ports on that element, and directing packets out through the port that leads in the direction of the target address. Routers and switches contain data paths.
The most common usage of the term data path refers to virtual switches where the control and data plane functions are clearly separated. In all network devices that support the open flow standard, the control and data paths are clearly separate. The first packet in a flow takes the slow path through the controller and updates the forwarding table in the data path before going through the data path. Subsequent packets in that flow take the fast path, which means entirely in the data path.