An aggregation of information system components that is designated for configuration management and treated as a single entity in the configuration management process.
Source(s):
NIST SP 800-128 under Configuration Item
NIST SP 800-53 Rev. 4 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.
Source(s):
NISTIR 7695 under Configuration Item from ISO/IEC 19770-2 - Adapted