Item or aggregation of hardware, software, or both, that is designated for configuration management and treated as a single entity in the configuration management process.
Sources:
NIST SP 800-160v1r1 from ISO/IEC/IEEE 15288:2015

  An aggregation of information system components that is designated for configuration management and treated as a single entity in the configuration management process.
Sources:
CNSSI 4009-2015
NIST SP 800-128
NIST SP 800-128 under Configuration Item

  An aggregation of system components that is designated for configuration management and treated as a single entity in the configuration management process.
Sources:
NIST SP 800-37 Rev. 2 from NIST SP 800-128
NIST SP 800-53 Rev. 5 from NIST SP 800-128

  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.
Sources:
NISTIR 7695 under Configuration Item from ISO/IEC 19770-2 - Adapted