Board Thread:Wiki/@comment-30771283-20170910044933/@comment-1327106-20170911071935

Empire can be countries. The German Empire is what Germany used to be called when it had a Kaiser.

Empires can also consist of multiple countires under a single leader, but because we know that there are Eldians who are unaffected by the Founding Titan, arguing whether the Eldia Empire was one country or a collection of many countries is not revelant. There are Eldians who are not Subjects of Ymir. Most of the time they will be one and the same, so it's okay to use the terms interchangeably, but there are a few instances like the Ackermans where it won't work.

By the way, if you just need the "but the manga says so" explanation, Chapter 96 is the one where they mention how the rulers are probably not Subjects of Ymir but another Eldian race.