I've noticed that one question was recently tagged filter. (It is the only question having this tag at the moment.)
I was thinking about a few related tags, which might perhaps be useful; but I wanted to ask about the opinion of other users before creating any of these tags. (I might be biased, since I am interested in these topics. And of course we have 5 tags limit, so we should not overtag.) And it was also advised here: If you are not sure whether a tag belongs, bring it to Meta.
Some arguments why I think these tags would be reasonable:
Aside note: Do you think it would be reasonable to make one big thread for questions related to proposed or newly-created tags, in the same way as we did for Tag merging and synonyms ?
EDIT: I believe 6 upvotes for Asaf's post can be understood as support for his suggestion, which was compactness tag and one common tag for filters and ultrafilters. He has already started compactness.
I've edited tag-wiki for filter. Now Asaf also created tag filters and added tag-wiki. I'll try to add a few questions in the next days, so that the tag is not automatically deleted for having no questions. (This will also have the effect that a few more people will be aware of this tag, since they will see it used on the main page.) I am not entirely sure about the preference of singular/plural in tag names; however I was not able to create filters tag, I've got the following warning: Creating the new tag [filters] is not allowed since the tag [filter] already exists. If you think this new tag should be allowed, raise it on meta.