An information system in which all three security objectives (i.e., confidentiality, integrity, and availability) are assigned a FIPS 199 potential impact value of low.
Sources:
FIPS 200 under LOW-IMPACT SYSTEM
NIST SP 800-60 Vol. 1 Rev. 1 under Low-Impact System from FIPS 200
NIST SP 800-60 Vol. 2 Rev. 1 under Low-Impact System from FIPS 200

  An information system in which all three security properties (i.e., confidentiality, integrity, and availability) are assigned a FIPS 199 potential impact value of low. Note: For National Security Systems, CNSSI No. 1253 assigns one impact value (low, moderate, or high) for each of the three security objectives (confidentiality, integrity, and availability), rather than using the FIPS 200 high water mark across security objectives. 
Sources:
CNSSI 4009-2022 from FIPS 200

  A system in which all three security objectives (i.e., confidentiality, integrity, and availability) are assigned a FIPS Publication 199 potential impact value of low.
Sources:
NIST SP 800-37 Rev. 2 from FIPS 200
NIST SP 800-53 Rev. 5 from FIPS 200
NIST SP 800-53A Rev. 5 from FIPS 200
NIST SP 800-53B from FIPS 200