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 design of MathOverflow. This includes bug reports and suggestions about the user interface.
55 questions
designed to solicit opinions or best-practices on a particular topic, with the goal of reaching community consensus.
8 questions
aimed for discussions about etiquette: how to behave on the site and the chat.
51 questions
8 questions
For questions about exporting and printing MathOverflow posts.
2 questions
the most frequently asked and answered questions on Meta.
19 questions
Proposals for new entries for the FAQ.
4 questions
For questions about the favorite question feature or the favorite tags feature.
9 questions
0 questions
For questions about the flagging process, how flags are handled, etc. Flagging is the process of bringing a post to a community moderator's attention for any reason.
22 questions
For issues regarding the help center of MathOverflow.
11 questions
Questions about the hot questions list which appears on the main page and—truncated—on various sites in the Stack Exchange network (but not on this site), as well as its effect on MathOverflow.
1 question
For questions on usage of links at MathOverflow (both links to various parts of this site and to other sites).
For questions involving the "Ignored Tags" specified in a user's Preferences. Questions with these tags are dimmed or removed entirely from question lists.
1 question
For questions concerning using images in questions, image hosting, etc.
14 questions
0 questions
a user-generated list of related questions shown in the sidebar when viewing a question. Its content comes from questions that users have linked in the question, thei…
1 question
12 questions
For questions about Markdown, a lightweight markup language used across the entire Stack Exchange network for post formatting.
10 questions
Questions about the live Markdown preview of posts as they're being typed.
1 question
Questions about the Mathematics Educators StackExchange site.
4 questions
Bug reports and suggestions for the MathJax system used to compile basic LaTeX code on the site.
67 questions
For questions concerning MathOverflow as non-profit organization. (As opposed to questions concerning MathOverflow as a website.)
7 questions
0 questions
Questions related to the closely related website, https://math.stackexchange.com
46 questions
Questions that have been closed as duplicates may sometimes be merged by moderators. One question is retained as a stub while all answers appear on the other.
2 questions
no Meta Meta Mathoverflow, so questions about this very site just get this tag.
13 questions
The process of migrating off-topic questions from one Stack Exchange site to another.
47 questions
Reproduction of significant parts of content of MO happens on some other sites. Some of them are covered by the license others are not. Questions on both types can be asked under this tag.