What is the definition for date line?

Answer 1

an imaginary line on the surface of the earth following (approximately) the 180th meridian

if you watch the suite life on deck theres an episode about it

Answer 2

The International Date Line is an imaginary line on the Earth's surface that determines the change in calendar date when crossed. It roughly follows the 180° meridian and is used to mark the transition between consecutive calendar days. Crossing the date line from east to west results in gaining a day, while crossing from west to east results in losing a day.