A blacklist in cybersecurity is a list of entities - such as IP addresses, domains, email addresses, or applications - that are known or suspected to be malicious, untrustworthy, or otherwise unwanted. Systems use blacklists to block or restrict access to these entities, preventing potential security threats like malware, spam, or unauthorized access. Blacklists are commonly used in firewalls, email filters, and web browsers to enhance security by denying communication with sources that have been identified as harmful. While effective, blacklists require constant updating, as new threats frequently emerge and attackers often change tactics to bypass existing lists.