When two lines are crossed by another line (Transversal):
Alternate Interior Angles are a pair of angles on the inner side of each of those 2 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 Interior of the two crossed lines.
When the two lines being crossed are Parallel Lines the Alternate Interior Angles are equal.