Definition of This Postulate?

If two lines in a plane are cut by a transversal so that a pair of alternate exterior angles is congruent, then the two lines are parallel.
In the image, if you know/prove ∠1 ≅ ∠8 OR ∠2 ≅ ∠7, then lines a and m are parallel.

 

See the image and this definition here:

https://quizlet.com/165214560/geometry-theorems-properties-etc-flash-cards/

 

Note: Scroll down about 3/4 of the way until you see " converse of alternate exterior angles postulate?"