Today I looked at the four "alternate_X" tags: alternate-earth, alternate-history, alternate-reality, alternate-worlds.
From my point of view, one or two of these could be different, but as they read in their descritpions, they're almost all the same thing:
alternate-earth This tag is for questions relating to radically changed Earth. This alternate Earth could be a post-apocalyptic Earth, or Earth with major changes in the current social/political/economical schemes.
OK, that's our planet with changes to lifestyle or socio-political structures.
alternate-history For questions that ask what might have happened if history had taken a different path.
OK, that's our planet with changes to lifestyle or socio-political structures.
alternate-reality For questions in which reality is different than it is in real life. This may range from small, historical changes to absolute revisions of the laws of physics.
OK, that's our planet with changes to lifestyle or socio-political structures.
alternate-worlds For questions about worlds other than Earth that are significantly different. This could be a water, ice, barren or other planet. For questions about planets similar to Earth with specific differences use the earth-like tag.
This is the only one that's different. But, honestly, I can't think of a reason to use it. It would apply to every question about a planet that isn't specifically about Earth. I'm sure we have plenty of other tags that are better suited to that purpose.
Question: Can we either (a) make some of these synonyms of each other, (b) use some better than they are being used, or (c) burn one or more of them?
Ultimately, this might be better suited to four separate questions, but I wanted everyone to see the big picture first.
I have posted my recommendations as answers per Vincent's suggestion.