The Fifth Postulate

The Fifth Postulate of Euclidean geometry, also known as the parallel postulate, states that if a straight line intersects two other lines so that the interior angles on one side are less than 180 degrees, then the two lines will eventually meet on that side if extended far enough. In simpler terms, it explains how parallel lines behave: if they are cut by a transversal and are not parallel, they will intersect on one side; if they are parallel, they never meet regardless of how far they are extended. This postulate is fundamental in defining the nature of parallel lines in classical geometry.