The individual, group, or organization responsible for conducting a security control assessment.
Sources:
CNSSI 4009-2015
NIST SP 800-137 under Security Control Assessor
NIST SP 800-30 Rev. 1 under Security Control Assessor
NIST SP 800-39 under Security Control Assessor

  See Security Control Assessor.
Sources:
NIST SP 800-137 under Assessor
NIST SP 800-172 under assessor
NIST SP 800-39 under Assessor
NIST SP 800-171 Rev. 2 under assessor

  The individual responsible for conducting assessment activities under the guidance and direction of a Designated Authorizing Official. The Assessor is a 3rd party.
Sources:
NIST SP 800-79-2 under Assessor

  The individual, group, or organization responsible for conducting a security or privacy assessment.
Sources:
NIST SP 800-37 Rev. 2 under assessor

  See security control assessor or risk assessor.
Sources:
CNSSI 4009-2015 under assessor from NIST SP 800-30 Rev. 1
NIST SP 800-30 Rev. 1 under Assessor

  The individual, group, or organization responsible for conducting a security or privacy control assessment.
Sources:
NIST SP 800-53 Rev. 5 under assessor

  The individual, group, or organization responsible for conducting a security or privacy control assessment.
Sources:
NIST SP 800-53A Rev. 5 under Assessor