Essos is the largest of the four known continents in the Known World. As its name implies, it lies to the east of Westeros and north of Sothoryos and Ulthos, stretching from the Shivering Sea in the north to the Summer Sea in the south, and from the Narrow Sea in the west to the lands surrounding the Jade Sea in the east, if not farther. Essos is populated by many different peoples and has a vast and varied geography. However, unlike Westeros, the majority of the content is not governed via a single autonomous state, and many city-states, nomadic tribes and ruins dot the gigantic landmass.
The term "Essos" actually wasn't used to describe the continent until a mid-2008 interview: in the A Song of Ice and Fire novels, it was simply referred to as the eastern continent, beyond the Narrow Sea or the east.