A blacklist is a list of items that are specifically identified and blocked or excluded from a particular system or process. In the context of online advertising, a blacklist is a list of websites, domains, or other online resources that are blocked or excluded from an ad campaign.

Blacklists are often used by advertisers and ad agencies to prevent their ads from being shown on certain types of websites or domains that they do not want to associate with. For example, an advertiser may use a blacklist to exclude their ads from being shown on websites that contain inappropriate or offensive content, or that are known to traffic in fraudulent or malicious activity.

Blacklists may be created and managed manually by advertisers or ad agencies, or they may be generated automatically by ad tracking and analytics tools. Blacklists may be used in conjunction with whitelists, which are lists of items that are specifically included or approved for a particular system or process.