The antimeridian, or 180th meridian, is the meridian which is 180° east or west of the Prime Meridian with which it forms a great circle. It is distinctive among meridians in that it is common to both east longitude and west longitude.
Many geographic software libraries or data formats project the world to a rectangle; very often this rectangle is split exactly at the 180th meridian. This often makes it impossible to do simple tasks (like representing an area, or a line) over the 180th meridian.
The antimeridian is used as the basis for the International Date Line because it for the most part passes through the open waters of the Pacific Ocean. However, the meridian passes through Russia and Fiji as well as Antarctica. The antimeridian and the International Date Line are not the same.