Function: noun
Date: 1610
: one of a pair of angles with different vertices and on opposite sides of a transversal at its intersection with two other lines: a : one of a pair of angles inside the two intersected lines ― called also alternate interior angle b : one of a pair of angles outside the two intersected lines ― called also alternate exterior angle
alternate interior angles a, a ′ , b, b ′ ; alternate exterior angles c, c ′ , d, d ′