“Software-defined storage” (SDS) is one of those terms where if you ask ten people for the definition, you’ll get at least eight different answers. It clearly creates some confusion (is that an oxymoron?). The biggest reason for the confusion is that it’s a term that’s still evolving. Hence, it changes on a regular basis.
At its simplest form, SDS is a separation of control-plane operations from the storage array or appliance. But that’s my definition, and I’m one of those ten people.
To try and put an end to the confusion, Dell has penned an article called What software-defined storage really means. I suggest you give it a read.