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.
The definition of SDN that is currently emerging focuses somewhat less on decoupling and more on providing programmatic interfaces into network equipment, whether or not there is a separation of the control and forwarding planes. A minor reason for this shift in focus is because Cisco recently announced that as part of its SDN offerings, it will provide APIs into multiple platforms that they provide.