SDN !!??

I was wondering lately about this word  "SDN" Software Defined Networking ,what is that ?? are we talking about NSX ? I figure out this.

Source :https://packetlife.net/blog/2013/may/2/what-hell-sdn/

The term "software-defined" was probably not the best choice to convey this concept, since routers and switches of course already run software like any other reasonably complex computer system. "Controller-defined" or "centralized control" would in my opinion have been more appropriate, but I suppose it's too late to change it now.)

Why SDN?

So what could we gain by migrating from a distributed control plane to a centralized one? One major benefit of centralizing the control plane is allowing forwarding decisions to be made globally across the SDN domain rather than at each hop. For example, imagine a layer two switching topology with redundant paths. Normally, spanning tree (a control plane process) is needed to guard against loops because no individual switch knows what the entire network looks like. However, if the control plane functions for all switches are offloaded to a central controller, that controller can "see" the entire network and install forwarding decisions to each subordinate switch based upon the desired end-to-end path for each destination or flow while keeping all links active.Another advantage of having a central controller is that we can provide a convenient programming interface to allow other applications to control network resources and influence forwarding decisions. As a hypothetical example, when a virtual machine is moved from one physical host to another, the controller could be automatically instructed to migrate any associated firewall or QoS policies (which are enforced in the forwarding plane) on the network side along with it. This removes the need for a human administrator to statically reconfigure network resources and allows the entire network to operate more fluidly.