From Dictionary.com:
God: the one Supreme Being, the creator and ruler of the universe.
I have no problem with that definition. It's clear and succinct.
The fact that God is imaginary, that there is no "one supreme being," no "creator and ruler of the universe," does not invalidate the definition.
The words "fairy," "leprechaun," "demon," "angel" and "hobbit" are also in the dictionary and those definitions are also accurate, clear and succinct. The fact that those referents are also imaginary doesn't mean that the definitions are wrong. Heck, "Klingon" is in the dictionary, and I'm pretty sure that
no one believes they are real. In this case the dictionary has the courage to say so.
I daresay that no one more than eight years old believes that the Easter Bunny is real, but we all agree on what the name means.
My personal definition of "dog" is "a creature more honorable than man, who can be depended on to be loyal, protective and loving until the day he dies," but if a stranger or someone on a science website asks me for a definition, this is not the one I will give him.
If someone asked me what a hobbit is, I would do my best to give a faithful descripion of the creatures in
Lord of the Rings, and only if necessary would I append, "but they're totally fictional."
If someone from Mars asked me what God is, I would do the same. Well... I suppose I would have to digress and explain that in some cultures, particularly the ancient ones, people believe/believed in multiple gods.