Greek pizza can refer to either of two styles of pizza

Pizza

Pizza is an oven-baked, flat, disc-shaped bread typically topped with a tomato sauce, cheese and various toppings.Originating in Italy, from the Neapolitan cuisine, the dish has become popular in many parts of the world. An establishment that makes and sells pizzas is called a "pizzeria"...

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One usage refers to a pizza with typically (or stereotypically) Greek ingredients as toppings. These include authentic toppings such as feta cheese, onion

Onion

The onion , also known as the bulb onion, common onion and garden onion, is the most widely cultivated species of the genus Allium. The genus Allium also contains a number of other species variously referred to as onions and cultivated for food, such as the Japanese bunching onion The onion...

, Kalamata

Kalamata

Kalamata is the second-largest city of the Peloponnese in southern Greece. The capital and chief port of the Messenia prefecture, it lies along the Nedon River at the head of the Messenian Gulf...

 olives, fresh tomato

Tomato

The word "tomato" may refer to the plant or the edible, typically red, fruit which it bears. Originating in South America, the tomato was spread around the world following the Spanish colonization of the Americas, and its many varieties are now widely grown, often in greenhouses in cooler...

, green bell pepper

Bell pepper

Bell pepper, also known as sweet pepper or a pepper and capsicum , is a cultivar group of the species Capsicum annuum . Cultivars of the plant produce fruits in different colors, including red, yellow, orange and green. Bell peppers are sometimes grouped with less pungent pepper varieties as...

, gyros meat and spinach

Spinach

Spinach is an edible flowering plant in the family of Amaranthaceae. It is native to central and southwestern Asia. It is an annual plant , which grows to a height of up to 30 cm. Spinach may survive over winter in temperate regions...

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The other usage refers to a style of pizza crust rather than its toppings. This style is baked in a pan, instead of directly on the bricks of the pizza oven (as is more traditional for Italian pizza.) It has a crust that is usually quite oily. This style of pizza is referred to as "Greek pizza" even when it has non-Greek toppings, since it is typical of pizzerias owned by Greek immigrants. These establishments often also sell Greek specialities, such as Greek salad

Greek salad

Greek salad is a summer salad in Greek cuisine.Greek salad is made with pieces of tomatoes, sliced cucumber, green bell peppers, red onion, sliced or cubed feta cheese, and Kalamata olives, typically seasoned with salt and dried oregano, and dressed with olive oil...

s and gyros, and tend to brand themselves as "Pizza and Pasta" or as a "House of Pizza"; a code signifying that it is not an Italian restaurant, but a Greek restaurant serving Italian style food.

In the United States, the latter usage appears to be specific to New England cuisine.