An aggregation of information system components that is designated for configuration management and treated as a single entity in the configuration management process.
NIST SP 800-128 under Configuration Management
A collection of activities focused on establishing and maintaining the integrity of information technology products and information systems, through control of processes for initializing, changing, and monitoring the configurations of those products and systems throughout the system development life cycle.
NIST SP 800-53 Rev. 4 under Configuration Management