1. An entity authorized by the certification authority system (CAS) to collect, verify, and submit information provided by potential Subscribers which is to be entered into public key certificates. The term RA refers to hardware, software, and individuals that collectively perform this function.
Sources:
CNSSI 4009-2015 from CNSSI 1300

  2. The key management entity (KME) within each Service or Agency responsible for registering KMEs and assigning electronic key management system (EKMS) IDs to them.
Sources:
CNSSI 4009-2015 from CNSSI 4005

  A trusted entity that establishes and vouches for the identity of a user.
Sources:
NIST SP 800-57 Part 1 Rev. 5 under Registration authority

  An entity authorized by the certification authority system (CAS) to collect, verify, and submit information provided by potential subscribers, which is to be entered into public key certificates. The term RA refers to hardware, software, and individuals that collectively perform this function.
Sources:
NIST SP 1800-16B under Registration Authority (RA) from CNSSI 4009-2015
NIST SP 1800-16C under Registration Authority (RA) from CNSSI 4009-2015
NIST SP 1800-16D under Registration Authority from CNSSI 4009-2015

  An organization approved by ISO/IEC for performing registration.
Sources:
NISTIR 5308 under Registration Authority from ISO/IEC JTC1 N820

  A trusted entity that establishes and vouches for the identity or attributes of a subscriber to a CSP. The RA may be an integral part of a CSP, or it may be independent of a CSP, but it has a relationship to the CSP(s).
Sources:
NISTIR 8149 under Registration Authority (RA) from NIST SP 800-63-3