Corn is the term for the native or common grain of a place. In the US it is maize, In other places it is wheat, in other places rye, or barley, or oats. we in US sometimes forget that-- and think corn as being specific to maize

I was suprised to read this, and upon LIU have my doubts.

Per bartleby, each of the grain-names noted refers to a is a distinct genus:
corn: 1a. Any of numerous cultivated forms of a widely grown, usually tall annual cereal grass (Zea mays)
wheat: Any of various annual cereal grasses of the genus Triticum ... especially T. aestivum
rye: 1. A cereal grass (Secale cereale) widely cultivated for its grain.
barley: 1. A grass in the genus Hordeum, ... 2. The grain of H. vulgare or its varieties,
oat: 1. often oats ... a. Any of various grasses of the genus Avena, especially A. sativa, widely cultivated for their edible grains.

Bartleby does also give the above-blue definition for "corn", but only as a teriary and "chiefly British" meaning.