Per-site lists of tags that cannot be created.
Tags that appear on the blacklist fall into any number of categories:
redundant - tags that repeat a site's scope, name or focus add nothing to the categorisation. An example would be the zoo tag on a site about zoos.
meta tags - tags should define the content of the question and not its character. A long fallen one, subjective is an example of such.
Blacklisted tags are blocked from being created. Blacklisting does not mean that the tag is automatically removed from the site - although it is often done together with burnination, which removes all instances of the tag.