Full Idea
Euclid's fifth 'parallel' postulate says if there is an infinite straight line and a point, then there is only one straight line through the point which won't intersect the first line. This axiom is independent of Euclid's first four (agreed) axioms.
Gist of Idea
Euclid's parallel postulate defines unique non-intersecting parallel lines
Source
report of Euclid (Elements of Geometry [c.290 BCE]) by Michèle Friend - Introducing the Philosophy of Mathematics 2.2
Book Reference
Friend,Michèle: 'Introducing the Philosophy of Mathematics' [Acumen 2007], p.24
A Reaction
This postulate was challenged in the nineteenth century, which was a major landmark in the development of modern relativist views of knowledge.