How to find out if a tag is good to create or have?

Question

A tag is a word or phrase that describes the topic of the question. Tags are a means of connecting experts with questions they will be able to answer by sorting questions into specific, well-defined categories. [Source: help/tagging]

Almost every alternate day, I come across users who create a ton of new tags whenever they post a new question. It's necessary to remind ourselves that not all tags are useful in the long run. Getting rid of bad tags is hard and the process often unnecessarily bumps old questions to the front page!

I had once mentioned:

As a rule of thumb, avoid creating tags which, if used as a stand-alone tag on a question, would probably make the question unsuitable for this site.

and I believe that's still a useful rule of thumb. Especially so, for meta tags.

Anyway, I'd like to summarize some more important points about tags and tagging in an answer below.

Answer 1

Here's a 12-point test for creating a new tag, say X. It is mostly an extension of the MAR Tag Test™!

1. Does X have a universally unambiguous meaning?
2. Does there exist a published paper (in a reputable venue) which defines X? Does X have a corresponding Wikipedia page?
3. Does "experts on X" make sense?
4. Does "I'm studying X" make sense?
5. Would you leave and not retag a question with only X as its tag?
6. Will "because it's related to X" be the only reason a question is tagged with X?¹
7. Will X not be misinterpreted, just judging by its name?
8. Are there many (>15) but not too many² questions on the site with the search keyword "X"?
9. Will people search using "X" to find questions to answer?
10. Without any personal feelings towards X, will people add it to their "ignored" or "favorite" tags?
11. Can the questions to which X can be applied have other tags reasonably applied as well?
12. Is X not a tag that can really only be applied once there are answers to its questions?

For each point that you fully agree with, give the tag a score of $\color{green}{+2}$. For each point that you partially agree with, give the tag a score of $\color{orange}{+1}$. And for each point that you don't agree with, give the tag a score of $\color{red}{-1}$. Find the $\text{total score}$. If

• $\color{green}{\text{total score}>18}$, then it's an exceptionally good tag. Go ahead and create it, if it doesn't exist already!
• $\color{orange}{18>\text{total score}>14}$, then it's a good tag. Make a judgement call and decide whether or not to create it.
• $\color{red}{\text{total score}<14}$, then it's not a good tag. Make sure to ask on meta before creating it, or it already exists make a meta post to ask for its possible burnination. Read: When to burninate.

_{1: Ignore spam and be aware that meta tags are dead. 2: A tag should be a tag, not more, not less. Ideally, a tag should be present only be on less than $\frac{1}{10}$th of an SE site's questions.}

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Important Note: For God's sake, please please please write up the tag excerpt when you create a new tag. Don't leave it for others! It's okay to copy over the corresponding definition from Wikipedia or some published paper, but in that case make sure you mention the source in brackets - something like (Wikipedia) or (arXiv:xxxx.yyyy) would suffice. It's always preferable that you write the definitions in your own words but I do realize that that can sometimes be time-consuming. However, do keep in mind that tag excerpts should ideally contain not only the definitions but also guidance on how and when to use them.