The imaginary line that divides the Earth into Eastern and Western Hemispheres is called the Prime Meridian. It is an important reference point used in geography and navigation, located at 0 degrees longitude. The Prime Meridian runs through Greenwich, London, and serves as the basis for the world's time zones. The line helps in understanding global positioning, geography, and the organization of maps. While it divides the Earth into two hemispheres, it works in conjunction with the International Date Line, which demarcates the change of calendar days and time zones.