One fascinating thing about the English language is its malleability -- the ability to adapt and adjust to changing times and different cultures. Just the differences between American English and variants spoken elsewhere are an absorbing study. The squash called zucchini here is a courgette there, the herb cilantro is called fresh coriander, and American corn is referred to more specifically in the U.K. as maize.
Etymology of Corn
The root of the English word corn is the proto-Germanic "kurnam," which also evolved across a period of centuries into the words kernel and grain. The word has traditionally been used to denote the primary grain crop in a given area, in the same way that "fish," if not otherwise specified, denoted cod in parts of New England and Atlantic Canada. In England the word was generally used to denote wheat, while in Scotland it was oats and in parts of Germany "korn" was understood to mean rye.
Maize was domesticated over 7,000 years ago in what is now Mexico from a wild grass called teosinte. Maize is vastly easier to cultivate than other grains, requiring no laborious cutting and threshing. Its cultivation spread throughout Central and North America, where Columbus discovered it and brought it back to Europe. The name maize originated with the indigenous Taino people in the Caribbean, and is the name used in most European languages to denote "Indian corn," or grain from the New World.
Corn in America
The earliest Spanish settlers and conquistadors learned to grow and appreciate corn as a source of food and fodder, and its cultivation in Latin America continued unchecked, unlike other native crops such as amaranth and quinoa. Maize quickly became an important crop in New England as well, flourishing for centuries before wheat farming was successfully established. The settlers originally knew maize as "Indian corn," but eventually conformed to the tradition of referring to the locally dominant grain crop simply as corn.
Other Corn Terms
The use of the word corn as a generic term for grains of any kind has survived into the modern era. Aficionados of folk music will know the ballad "John Barleycorn must die," a tongue-in-cheek tale of the grain's life and conversion into ale. Whole berries of black, white or green pepper are still referred to as peppercorns, because of their grain-like appearance. One of the odder survivals is the term "corned beef," which on the surface would appear to have nothing grain-like about it. The term actually refers to the grains of coarse salt used in the curing process.