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.

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available to users with a reputation of 10000.
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the answer the question owner thought best met their needs. Accepting is encouraged but never required. This tag is for *discussing* issues related to accepted answers, not indi…
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the percentage of questions for which the user has accepted an answer.
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the act of temporarily blocking the account of a specific user.
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when asking what is on- or off-topic on the main site, and why.
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for questions about the official Stack Exchange Android App
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for questions about answers on the main site. Consider using the accepted-answer tag if the question is about an answer that has been accepted.
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the Stack Exchange Network staging zone, where users come together to build new Q&A sites. New site ideas are proposed, discussed, and the best go on to beta.
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Questions specifically about the process of asking questions on this site.
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a process which helps cleaning the site by deleting some questions. Criteria for auto-deletion are based on low score and age of the question.
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awarded to encourage and incentivize positive community actions within the site.
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For questions regarding the badge tracking functionality in the user profile area.
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For questions about the 'being nice' behavior we expect from all Stack Exchange users. What this entails can be found in the help center on every site.
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should be used when you are expecting a large number of diverse answers to a question.
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For questions about the words and phrases which are blocked from appearing in posts or titles.
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From Oxford Dictionaries, paraphrasing to suit the needs of *this site*: "A website or web page on which (in our case) M.SE records opinions, links to other sites, etc. on a regular basis".
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a way to get additional attention for a question by offering some of your own reputation for great answers.
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Indicates a reproducible problem on the site that you believe is due to a mistake, malfunction, or programming error.
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Some changes to questions, such as edits or receiving new answers, will "bump" them back onto the site's front page where the changes can be vetted by other users.
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distinguish humans from bots.
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For questions that have as their purpose the celebration of a milestone, either for a specific user or the site.
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Ceiling and floor of the number of characters you can enter into various slots
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For questions about real time web chat that accompanies Mathematics Stack Exchange site.
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For questions concerning citation posts from MSE and citation of other resources when posting here.
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For instructions and guidance on closing or migrating questions, and the reasons for those closures.
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The process by which users can vote a question closed if it is off-topic or otherwise disallowed by the FAQ.
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small addendums to each question or answer, intentionally limited in size and formatting.
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For questions about comment @replies (a.k.a. pings).