There are 2: The Equator & the Prime (Greenwich) Meridian.
The Equator
The Equator
All parallels, or latitudes, cross 0 degrees longitude. 0 degrees longitude is the prime meridian. 0 degrees latitude, or parallel, is the equator
The 0 degrees is the planet Earth equator. The 50 degrees is the parallel [geographic latitude] that crosses several countries but the most importand is Belgium where the capital of EU is located
The Prime Meridian, it is located at 0 degrees latitude.
That's the equator.
the prime meridian
The equator is located at 0 degrees latitude. The angle of latitude starts at 0 at the equator and ends with +90 and -90 degrees at the North and South poles, respectively. Lines of latitude are parallel to each other.
E q u a t o r
It is the equator.
Galapagos
Prime meridian