When 2 lines are crossed by another line (Transversal):
Alternate Exterior Angles are a pair of angles on the outer side of each of those two lines but on opposite sides of the transversal.
How to remember
the angle pairs are on Alternate sides of the Transversal, and they are on the Exterior of the two crossed lines.
When the two lines being crossed are Parallel Lines the Alternate Exterior Angles are equal.