Final answer:
The International Date Line is the imaginary line that runs 180 degrees away from the Prime Meridian and marks where one calendar day ends and another starts.
Step-by-step explanation:
The imaginary line that runs from the North Pole to the South Pole and is 180 degrees away from the Prime Meridian is called the International Date Line. It marks where one calendar day ends and another starts. The International Date Line runs approximately along the 180-degree meridian of longitude, down the middle of the Pacific Ocean, but it deviates in some places to accommodate political agreements.