How could the Romans use corn? It's American!

Check out The Straight Dope for the answer (a portion is pasted below)…

"Corn" comes from the Latin word for grain (granum), and through the ages it's been used indiscriminately for whatever grain happens to predominate in a particular region.

Maize is, of course, a product of the New World. No historical evidence suggests that any European had encountered it before Christopher Columbus landed in Cuba. According to Columbus's journal for that fateful day, November 5, 1492, two Spanish scouts he had sent to explore the interior of the island came back with wild tales of "a sort of grain . . . which was well tasted, baked, dried, and made into flour." The natives, in their Taino dialect, called it mahiz, which Columbus promptly corrupted into maiz or maize.