Original: In geometry, the parallel postulate, also called Euclid's fifth postulate because it is the fifth postulate in Euclid's Elements, is a distinctive axiom in Euclidean geometry.
Simplified: In geometry the Parallel postulate is one of the axioms of Euclidean geometry. Sometimes it is also called Euclid's fifth postulate, because it is the fifth postulate in Euclid's Elements.