A tag is a keyword or label that categorizes your question with other, similar questions. Using the right tags makes it easier for others to find and answer your question.
For questions about the user named Community ♦ present on MathOverflow (and other Stack Exchange sites.)
an editable page that briefly summarizes the topic of the tag and that may provide links to existing questions that are often useful to many people. Sometimes tag-wiki also includes short …
For questions which focus on a particular user, rather than those discussing general user issues.
Questions about the tools available for users with at least 10000 points of reputation.
those questions which have neither an accepted answer nor answers with positive vote count. Note that our definition is strict!
For questions about user accounts that have been deleted, or the general process of account deletion.
For questions concerning MathOverflow as non-profit organization. (As opposed to questions concerning MathOverflow as a website.)
For questions about arXiv trackbacks.
the name currently used for the old meta site of MathOverflow, which is now hosted on http://mathoverflow.tqft.net/ In contrast to the current site it was not Q&A based.
For questions about bonus reputation obtained when associating SE accounts.
Questions about the Mathematics Educators StackExchange site.
For questions about citation helper feature.
For discussions about blacklisted tags and proposals to blacklist some tags.
the act of casting many successive upvotes or downvotes on the posts of a specific user.
Questions related to the "side bar"; the bar that runs down the right side of the page on Stack Exchange sites.
For questions about the words and phrases which are blocked from appearing in posts or titles.
A message displayed to alert the user to a problem or potential problem.
The Stack Exchange Data Explorer (SEDE) allows you to create and run queries against copies of the databases of popular Stack Exchange sites.
for sandbox questions, which allow people to write answers meant for testing of a particular issue, or a draft for a later answer.
For questions about the various RSS feeds provided by Stack Exchange sites.
an open standard that allows users to log in to many websites with a single set of third-party credentials.