Words or phrases that are not allowed to appear in given content and are an indication the content is likely not releasable to the destination domain. To be contrasted with clean words, those words that contain a dirty word but are allowed to appear in given content (e.g. if "secret" was a dirty word, then "secretary" could be a clean word).
Sources:
CNSSI 4009-2022 from Raise the Bar Baseline Release

  A blacklist is a list of discrete entities, such as hosts, email addresses, network port numbers, runtime processes, or applications, that have been previously determined to be associated with malicious activity. Note: Blacklist is also known by the terms “block list/blocklist” and “deny list/denylist.”
Sources:
CNSSI 4009-2022 under blacklist from NIST SP 800-167 - adapted

  A list of discrete entities that have been previously determined to be associated with malicious activity.
Sources:
NIST SP 800-167 under blacklist