From IDC: storage software stack that can be installed on any commodity resources (x86 hardware, hypervisors,cloud) and/or off-the-shelf computing hardware and used to offer a full suite of storage services and federation between the underlying persistent data placement resources to enable data mobility of its tenants between these resources.
From Gartner: software approach that abstracts storage capabilities dynamically derived from the physical and virtual storage devices and/or services, regardless of location or class of storage, to improve agility and deliver quality of service (QoS), while optimizing cost. Storage services can be orchestrated via interoperable real-time programmable interfaces through the software layer that are separated into the control plane and the data planes. SDS orchestrates storage services, independent of where data is placed and how it is stored, through software that, in turn, will translate those capabilities into storage services that meat a defined policy or SLA.
From SNIA: Software Defined Storage has the following attributes: May allow customers to “build it themselves,”providing their own commodity hardware to create a solution with the provided software. May work with either arbitrary hardware or may also enhance the existing functions of specialized hardware. May also enable the scale-out of storage (not just the scale up typical of big storage boxes). Nearly always includes the pooling of storage and other resources. May allow for the building of the storage and data services “solution” incrementally. Incorporates management automation. Includes a self service interface for users. Includes a form of service level management that allows for the tagging of metadata to drive the type of torage and data services applied. The granularity may be large to start, but is expected to move to a finer grained service level capability over time. Allows administrators to set policy for managing the storage
Your own definition: