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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 Item
NIST SP 800-53 Rev. 4 [Superseded] under Configuration Item
An item or aggregation of hardware or software or both that is designed to be managed as a single entity. Configuration items may vary widely in complexity, size and type, ranging from an entire system including all hardware, software and documentation, to a single module, a minor hardware component or a single software package.
NISTIR 7695 under Configuration Item from - Adapted