Greenwich
The International Date Line.
The international date line zig-zags to avoid intersecting land.
Greenwich Meridian is a line of longatude.
The lines of longitude are also known as a meridian line.
The major line of longitude located at 180 degrees longitude is the International Date Line.
The Prime Meridian is a line of longitude, and it sits at 0 degrees longitude.
It is a MERIDIAN. Its meridial line is is 180 degrees. , exactly on the opposite side of the Earth to the 0(zero) meridian. However, the I.D.L. does have a number of 'kinks' in it, to accommodate local topography.
The International Date Line roughly follows the 180th meridian.
Greenwich is a place in England. If you look at a map, places to the left are west of Greenwich and places to the right are east. A meridian is any line which runs from north pole to south pole and the meridian that runs through Greenwich is called the Greenwich Meridian and by definition is zero degrees. All other meridians are measured in degrees from zero. E.g. New York is -74 degrees, meaning it's west of Greenwich by 74 degrees.
The line of latitude found at 0 degrees is called the equator. It is an imaginary line that divides the Earth into the Northern and Southern Hemispheres.
This is the "prime meridian" (also known as the Greenwich Meridian for the British observatory on the line).