What is the meaning of the word Adath?

Since you don't give any details of where you know this Word from, and in what language it is, I'll try to tell you what I know and hope that's what you wanted. I know of a Hebrew Word "Adah" (pronounced "Eidah"), which means "Congregation" (usually in a religious context). It can also mean a group of people from the same place or practising the same customs - religious or otherwise. The "th" at the end of the Word is grammatical, and is used whenever there is a noun after the term "Adath", like in "Congregation Shalo'm", which would be "Adath Shalo'm", or any other name, whatever the name of that particular congregation is.