The Western Hemisphere or western hemisphere is mainly used as a geographical term for the half of the Earth that lies west of the IERS Reference Meridian (which crosses Greenwich, London, United Kingdom) and east of the Antimeridian, the other half being called the Eastern Hemisphere.

In this sense, the Western Hemisphere consists of the Americas, the western portions of Africa and Europe, the extreme eastern tip of Russia, numerous territories in Oceania, and a portion of Antarctica, while excluding some of the Aleutian Islands to the southwest of the Alaskan mainland.

The term is often used in political rhetoric to refer to only North America and South America (or the New World) and adjacent islands; however, the Western Hemisphere technically includes all of the aforementioned territories.

In an effort to define the Western Hemisphere as the parts of the world which are not part of the Old World, there also exist projections which use the 20th meridian west and the diametrically opposed 160th meridian east to define the hemisphere. This projection excludes the European and African mainlands and a small portion of northeast Greenland, but includes more of eastern Russia and Oceania (e.g., New Zealand).