A report (e.g., a list) of one or more keys that have been revoked and the date(s) of revocation, possibly along with the reason for their revocation. Certificate Revocation Lists (CRLs) and Compromised Key Lists (CKLs) are examples of RKNs, along with Online Certificate Status Protocol (OCSP) responses (see RFC 6960).
Sources:
NIST SP 800-57 Part 2 Rev.1