An Italian omelet that usually has the ingredients mixed with the eggs rather than being folded inside, as with a French omelet. It can be flipped or the top can be finished under a broiling unit. An omelet is cooked quickly over moderately high heat and, after folding, has a flat-sided half-oval shape. A frittata is firmer because it's cooked very slowly over low heat, and round because it isn't folded.
From The Food Lover's Companion, Fourth edition by Sharon Tyler Herbst and Ron Herbst. Copyright © 2007, 2001, 1995, 1990 by Barron's Educational Series, Inc.