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 every user, the reputation from the main site is shown on meta. However scores of the posts on meta are still stored, so it is possible to obtain reputation-based badges on meta (mortarboard, tag …
For issues with how Stack Exchange sites look or work that are specific to Firefox.
"The Dupehammer" refers to the ability of tag gold badge owners to unilaterally close and reopen duplicates which were asked in tags of which they have a golden badge.
For questions related to specific review audits that appear to be faulty or with which you disagree.
When viewing a question, the "Related Questions" block is shown on the right side of the page and provides a system-generated list of questions similar to the current one.
For questions about vandalism, the deliberate act of damaging a post.
For questions about posts that violate the SE "Be Nice" policy.
Score means the total number of upvotes minus the total number of downvotes.
For questions involving the "Ignored Tags" specified in a user's Preferences. Questions with these tags are dimmed or removed entirely from question lists.
Indicates a post used for submitting and voting on Community Promotion Ads.
For questions about the timeline view of questions, accessible using the clock icon next to the post or via "/posts/xxx/timeline".
For questions relating to the reputation tab in user profiles.
A message displayed to alert the user to a problem or potential problem.
For questions on the posting/quality/reviews of late answers.
Grace period is time within which actions can be revised without a trace in the system. This includes a period in which you can undo and change your vote; a period where editing posts can be done free…
For questions about various counters showing number of views of a particular question, profile page, ...
For questions about the "protected" in "protected question."
For questions discussing possibilities to privately contact other users.
For questions involving issues of how words or names are spelled.
For questions related to the support of HTTPS by Mathematics Stack Exchange, which is finally live as of spring 2017.
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.
For questions about timestamps that appear in notices and tooltips upon hovering those notices.
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.
Reverts a post to the previous edit version, if any exists.
For questions related to the sort orderings of questions, answers, and other lists on SE sites.
Friendly comparison chart showing the reputation gains and losses between users on a site
for questions regarding unregistered users.
"Community Bulletin" is the new block that replaces the "Visit Meta" block that was before used on Stack Exchange sites.