This question already has an answer here:
Consider, for a moment, a worthwhile technology like Dropbox.
If you want to use their API, you might have a question you would ask on Stack Overflow.
If you had a question about the Dropbox app that you install on your PC to automatically synchronize files to and from the cloud, you'd want to ask it at Super User.
If you were having an issue with the web interface, a question on Web Applications wouldn't go amiss.
Let's not forget there's a very nice Android app you might ask about on Android Enthusiasts.
Oh, right. There's an iOS app too, which you'd ask about at Ask Different.
That's how we do things around here, and it's all good. (I'm much better equipped to help someone with the Android app or the web interface than with the iOS app, for instance.) And, inevitably, there will be some cross-contamination: a question about the PC app on Web Apps, for instance.
Wouldn't it be nice to be able to cross-link those tags to all of those other sites? And to do so in a standardized way?
Yes, of course, tag wikis can be edited to include links to those other sites. But it's not standardized, so it will look different on every site, and it's a bit of effort.
So I propose a system similar to synonyms. Call it "cross-site synonyms" or something. Where someone can propose that a tag on one Stack Exchange site is the counterpart of a tag on another Stack Exchange site. Heck, even allow people to vote on those links like synonyms.