A blacklist is a list of people, organizations, or countries that are shunned or excluded by others because they are alleged to have engaged in unacceptable or unethical behavior or activities. Being on a blacklist can have serious ramifications, such as being barred from getting funding, doing business, or getting jobs. Blacklists can be created by different entities, including governments, individuals, and corporations. In computing, a blacklist is a basic access control mechanism that blocks access to all elements on the list, such as email addresses, users, passwords, URLs, IP addresses, domain names, and more). Blacklists can be applied at various points in a security architecture, such as a host, web proxy, DNS servers, email server, firewall, directory servers, or application authentication gateways). The opposite of a blacklist is a whitelist, allowlist, or passlist, which only allows through elements on the list).

In recent years, there has been controversy over the use of the terms "blacklist" and "whitelist" in computing industries, as some consider them insensitive). As a result, some companies, open-source communities, and software developers have chosen to deprecate the use of these terms in favor of names that describe the purpose of those lists less ambiguously and are unlikely to come across to a reader as insensitive, such as allow list and deny list).