a peninsula on Long Island. In the book The Great Gatsby it represents Old Money families. (it is just east of West Egg in the map below). Notice it's contemptuously looking down on West Egg.
map: by OpenStreetMap®, licensed under the Creative Commons Attribution-ShareAlike 2.0 license
Across the courtesy bay the white palaces of fashionable East Egg glittered along the water, and the history of the summer really begins on the evening I drove over there to have dinner with the Tom Buchanans.