Gan: Corn is defined as a small hard grain/seed
Wheat is corn
Rice is corn
Rye is Corn
Millet is Corn
Maize is also corn
The term Corn used in supermarkets is actually slang....
If you are going to be a vocab critic then at least get the vocab right!
Though "corn" has meant any kind of grain for most its history, it has now come to mean "maize" in America. This has been standard usage and not just slang for well over 100 years. If someone says "corn" in the US, it never means "wheat," "rye," "barley," "rice," or "millet." In an international context, it is most clear to use "maize," but to call "maize" "corn" in America is no less correct than to call a fluffy wheat bun leavened with baking soda a "biscuit."