To blacklist someone or something means to put them on a list of people or things to be avoided, excluded, or punished. When a company blacklists a supplier, they refuse to do business with them anymore, often because the supplier did something wrong or broke their trust. A school might blacklist a website, blocking it from their computers to protect students.

During the 1950s, Hollywood studios kept blacklists of writers, actors, and directors they refused to hire because of their political beliefs. This period in American history destroyed many careers unfairly.

Today, blacklisting can serve useful purposes: credit card companies blacklist stolen card numbers to prevent fraud, and email systems blacklist known spam sources. But blacklisting can also be misused when it's done unfairly or secretly, punishing people without giving them a chance to defend themselves.

The opposite of blacklisting is whitelisting, where you create a list of approved people or things, like a whitelist of safe websites kids can visit.