I've looked through a number of the belief questions, and I'm just not seeing it; "Aqidah" and "Belief" might be synonyms, but in this context it refers to a particular type of belief, not just any old thing that happens to be or could be believed.
For example, questions like Why in Islam there are prescriptions regarding health? but not Penicillin? and Should a Muslim believe in Dinosaurs? are tagged belief but they don't feel like aqeedah questions at all.
Rather than synonyming/merging these prematurely, I think the best course of action here is to figure out how they're even being used (and how or even if they should be used; i'm not entirely convinced belief is focussed enough to be worth keeping): Update the tag wikis accordingly, and clean up the outliers. Once we know how the tags are actually used and intended to be used, then we can revisit whether or not they're overlapping.
Can someone who's a native Arabic speaker (or close to it) confirm there's no problematic language nuance here? Wikipedia writes:
Aqidah... is an Islamic term meaning "creed".
A creed (also known as a confession, symbol, or statement of faith) is a statement of the shared beliefs of a religious community in the form of a fixed formula summarizing core tenets.
This seems okay.
This makes sense as per:
Using the proper vernacular is an important part of setting the right tone for the site. Certainly, questions involving simpler concepts are welcome, but you should not be targeting and reaching out to "curious onlookers" specifically. -- Robert Cartaino ♦
This change passes my sanity checks.
While we're at it, can we think up a tag excerpt for this tag? "...a short blurb that describes when and why a tag should be used on this site specifically." The Hinduism.SE belief tag excerpt is half-decent. Maybe we can start with the following adaptation and improve it over time:
For questions about the Islamic belief system, e.g. the afterlife, predestination, angels. Can be used in conjunction with denomination tags to limit the scope.
Update: Done now. I changed it to the above. Feel free to improve.