* Proper noun: When referring to a specific deity, like the God of the Abrahamic religions, "God" is a proper noun and should be capitalized.
* Common noun: When referring to a deity in a general sense, like "the god of war," "god" is a common noun and should not be capitalized.
Here are some examples:
* Proper noun: "I believe in God."
* Common noun: "The ancient Greeks worshipped many gods."
Therefore, the answer is that "God" can be both a common and a proper noun, depending on the context.