The Western Hemisphere refers to The Americas and nearby islands. The terminology is meant to serve as an analogy with the natural geographic division of the world into a Northern and Southern Hemisphere. However, it is more a geopolitical rather than a geographical term, and refers mostly to the governments and nations that are located in the region; there is no physically-based boundary on the planet that actually separates the Western Hemisphere from the rest of the world, such as the equator which is defined by the rotation of the earth and which thus separates the Northern and Southern hemispheres. The term Eastern Hemisphere tends to be used much less often than Western Hemisphere.
