The Narrow Sea is the body of water lying to the east of Westeros. It separates Westeros from the continent of Essos to the east, and runs from roughly east of White Harbour (where it empties into the Shivering Sea) to the Stepstones, which separate it from the Summer Sea.
The Narrow Sea is so-called because the amount of water separating the two continents is relatively constrained. Merchantmen and traders from Westeros, the Free Cities and Ibben, as well as points further east, criss-cross the sea, calling at ports such as King's Landing, Gulltown, Braavos and Pentos. Trade in the Narrow Sea is sometimes disrupted by warfare, most notably the near-constant feuds between Myr and Lys over the Stepstones and Disputed Lands. These skirmishes often involve the use of naval warfare.