Tags on SE can be made synonyms with or without merging.
Non-merged synonyms have a bit of a weird behaviour. Take a look e.g. at books which is a synonym of references. If I search for [books], I get redirected to the search for [references], and the questions tagged with [books] are included into the search, try https://stats.stackexchange.com/questions/tagged/books (880 results). If somebody writes a question and tags it with [books], it will automatically be re-tagged to [references] at the moment of posting. If somebody edits an old question tagged with [books], it will automatically be re-tagged to [references] too. If I see a question tagged with [books] and mouse-over this tag, I see tag wiki excerpt from [references].
- questions tagged with [books] are still displayed with [books], and
- if I browse https://stats.stackexchange.com/tags, I see [references] with only
× 713question count and [books] appears with
× 168question count and its own tag wiki excerpt (which can be seen pretty much nowhere else) --
and both of these things are somewhat annoying.
"Merging" simply means that all the questions with [books] are silently and instantly re-tagged to [references]. According to the meta.SE thread What are tag synonyms and merged tags? How do they work? (emphasis mine),
All good tag synonyms should eventually be merged (source). This retags all questions tagged with a synonym – which is not automatically done when a synonym is created. As merging is not (easily) reversible, it should be done with caution and only when a synonym is established. Only moderators can perform a merge.
So, shall we be merging all our "established" tag synonyms? How should we define when a synonym is "established" enough? I would say 1-3 months trial period after a synonym is created should be enough. If so, should we go over all existing synonyms and do the merges?
There is even a special tab for the un-merged ones on the tag synonyms page, clearly indicating that the SE system is inviting the mods to do the merges: