In geometry, Playfair's axiom is an axiom that can be used instead of the fifth postulate of Euclides (the Parallel postulate):
Given a line and a point not on it, at most one parallel to the given line can be drawn through the point.
It is equivalent to Euclid's parallel postulate and was named after the Scottish mathematician John Playfair. It is only required to state "at most" because the rest of the postulates will imply that there is exactly one. It could perfectly be assumed to write it saying "there is one and only one parallel". It is important to remark that in the Euclid book, two lines are said to be parallel if they never meet. It does not matter if their distance is always the same or not.[1][2]
When David Hilbert made his Hilbert's axioms he used Playfair's axiom instead of the original one from Euclid.[3]
This axiom is used not only in Euclidean geometry, but also in a broader study called affine geometry where the concept of parallelism is central. In the context of affine geometry the axiom has been called Euclid's parallel axiom,[4] but for Euclidean geometry the parallel postulate which refers to angles is the traditional expression of parallelism.