Why did the fifth postulate become known as the parallel postulate?

The fifth postulate of Euclid’s Elements became known as the parallel postulate because it provided the logical foundation for the uniqueness of parallel lines within his geometric system.1

While Euclid’s other four postulates were considered intuitive and self-evident, the fifth postulate—which describes the conditions under which two straight lines will eventually meet—was viewed as more complex and less obvious. Because it was used to prove the existence of a unique line parallel to a given line through a specific point, it became inextricably linked to the study of parallels.213

For centuries, mathematicians attempted to prove that this postulate could be derived from the other four, but these efforts ultimately led to the 19th-century discovery of non-Euclidean geometries, which demonstrated that the parallel postulate is not a necessary truth but rather a defining feature of Euclidean geometry.21