There are now 186 questions tagged back, and 215 tagged back-button. The latter has the synonyms backbutton and browser-back-button, too. None of them have tag wikis or excerpts.
All the questions tagged back also have the word button in it (at least the search
[back] -button gives 0 results), which seems to indicate that they really all are about a back button of some kind.
Thus I propose merging back into back-button, and declaring it a synonym.