This recent question has created the tag, "muslim-empire". This is a rather vague reference, just as "chirstian-empire" could refer to the Franks, the Byzantines, the Russians, the Spanish, etc. I think what the author of the tag was referencing was the early Islamic caliphates. I think it would be far more clear if we divided this tag into its respective caliphates: the Rashidun-Caliphate, Umayyad-Caliphate, and the Abassid-Caliphate.
I see you went ahead and did it. I approve. "Muslim Empire" doesn't really make any more sense than "Christian Empire" would.
I suppose I could see just one for "Caliphate". That would at least strongly imply the period where there was only one Caliph, rather than perhaps encompassing the Mughal Empire in India, like "muslim-empire" might.