Software-defined storage (SDS) is a system of abstracting data storage so that the provisioning and management of storage are separated from the underlying hardware. This allows separate pools of physical storage resources to be managed together as a single logical device.
The VMware Perspective on Software-Defined Storage - (Whitepaper)
Hyperconverged Infrastructure for Dummies, 2nd Edition
SDS enables increased automation and simplified management, standardized interfaces and greater scalability and transparency for monitoring and managing storage.