Software-defined infrastructure (SDI) is the definition of technical computing infrastructure entirely under the control of software with no operator or human intervention. It operates independent of any hardware-specific dependencies and is programmatically extensible.
The concept refers to the ability to define your application requirements from the infrastructure (both functional and non-functional requirements) and have physical implementation of the hardware require to deliver those requirements automatically derived and provisioned.
Typical deployments require SDN and Cloud capabilities as a minimal point of entry.
Advanced capabilities enable the transition from one configuration to another without downtime, by automatically calculating the set of state changes between one configuration and another and an automated transition step between each step, thus achieving the complete change via software.
Additional value from SDI approach is to have versioning of your landscape enabling rollback and cloning.