Per-site lists of tags that cannot be created or used. To suggest a tag be blocked, use [tag-block-request] instead.
Tags that are blocked fall into any number of categories:
redundant - tags that repeat a site's scope, name or focus add nothing to the categorization. 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.
Blocked tags are prevented from being created or used, and users who edit posts that already used them must edit the tag out. Blocking a tag 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.