What are alternate interior angles?

Two parallel lines and one transversal intersecting it form ##8## different angles.
Two pairs of these angles are called alternate interior.
See picture below.
A pair of angles ##a## and ##d## are alternate interior.
So is a pair of angles ##b## and ##c##.
They are called alternate because ##a## and ##d## (as well as ##b## and ##c##) lie on opposite sides from transversal ##l##.
They are called interior because both ##a## and ##d## (as well as ##b## and ##c##) are inside a space between the parallel lines.
Two angles forming an alternate interior pair are always congruent to each other they have exactly the same measure.