The prime meridian is what divides the Earth into Eastern and Western hemispheres. This is a line of longitude with a measurement of 0 degrees.
The Earth is a sphere that can be divided into two hemispheres, Eastern and Western. The Eastern Hemisphere is traditionally known as the "Old World," while the Western hemisphere is known as the "New World." This is not to be confused with the term Western world, however, as this cultural division is determined by countries and lifestyles.
There are four total hemispheres that the Earth can be divided into. This includes the Eastern and Western, as well as the Northern and Southern.