The testing or evaluation of security controls to determine the extent to which the controls are implemented correctly, operating as intended, and producing the desired outcome with respect to meeting the security requirements for an information system or organization, as defined in § 170.15 to § 170.18. (CMMC-custom term)
[32 CFR Part 170, § 170.4Acronyms and definitions]

The testing or evaluation of security controls to determine the extent to which the controls are implemented correctly, operating as intended, and producing the desired outcome with respect to meeting the security requirements for an information system or organization.
[NIST Glossary, assessment – Glossary | CSRC (nist.gov)]