As is somewhat typical of emerging technologies, there isn't a universally agreed to definition of what is meant by software-defined networking (SDN). Over the last year or two most of the definitions have focused on the decoupling of the network control plane from the network forwarding plane.
Decoupling of the network control and forwarding planes isn't a new concept. It's a key feature of MPLS and it is also a characteristic of many contemporary WiFi networks. However, if SDN is looked at strictly as the decoupling of the network control plane from the network forwarding plane, then its value is limited to features like reducing network latency.