Real Dictionary

pizza
variable noun: A pizza is a flat, round piece of dough covered with tomatoes, cheese, and other savoury food, and then baked in an oven.

pizza in American English
a baked Italian dish consisting of flattened bread dough covered variously with herbs, fresh vegetables, or, typically in the U.S., with tomato sauce, grated cheese, and, often, sausage, mushrooms, pepperoni, etc.
noun: a flat, open-faced baked pie of Italian origin, consisting of a thin layer of bread dough topped with spiced tomato sauce and cheese, often garnished with anchovies, sausage slices, mushrooms, etc

pizza in British English
noun: a dish of Italian origin consisting of a baked disc of dough covered with cheese and tomatoes, usually with the addition of mushrooms, anchovies, sausage, or ham