A Block List or Deny List is a list of entities that are blocked or denied privileges or access. Hosts or applications that have been previously determined to be associated with malicious activity are examples of entities that may be on a block or deny list. This is a basic control mechanism used by IT professionals that deny access to specific email addresses, IP addresses, URLs, usernames, passwords, etc.
Related Term: Allow List, Permit List, (Deprecated: Whitelist)
Source: DHS personnel, NIST SP 800-94