This is from research only, as I am no more a native to this land then you are, but it is how the language progressed from an old term. They seem to have named it after one of their old gods, and it is not the only name it carries. It has been named Gaia, and Terra, both names of Deities.
Then Wikipedia is a good start, on the basis that whatever sources you find there are ones you really need to study yourself as sometimes things can be, not incorrect but stated badly.