Final answer:
Co-interior angles are pairs of angles that are on the same side of a transversal crossing two parallel lines and not between the parallel lines. They are supplementary, meaning they add up to 180 degrees.
Step-by-step explanation:
When two parallel lines are intersected by a transversal, there are several types of angles formed. One of these types is a pair of angles that are on the same side of the transversal and are not between the parallel lines. These angles are called co-interior angles or same-side interior angles. Co-interior angles are supplementary, which means that they add up to 180 degrees.
For example, if we have two parallel lines crossed by a transversal, and one pair of co-interior angles measures 80 degrees, the other co-interior angle will measure 100 degrees since the sum of the angles is 180 degrees.