Definition of parallel postulate in US English:

parallel postulate


  • The fifth postulate of the first book of Euclid's Elements, which states that if two straight lines are cut by a third, and the sum of the interior angles on one side of the third line is less than two right angles, then the two straight lines meet on that side of the third, or (equivalently), given a straight line and a point not on the line, there exists only one straight line through the point parallel to the given line.


Late 19th century; earliest use found in American Journal of Mathematics.