Interior angles are a type of angle that are formed on the inside of two lines that intersect, commonly known as a transversal. When a transversal intersects two parallel lines, it forms eight angles. These angles can be classified into different categories, such as interior angles, exterior angles, alternate interior angles, alternate exterior angles, corresponding angles, and consecutive interior angles.

Definition of Interior Angles:
Interior angles are the angles formed on the inside of the two parallel lines when a transversal crosses them. They are located between the two parallel lines and are not adjacent to the transversal. In other words, they are the angles that are formed on the same side of the transversal, between the parallel lines.

Characteristics of Interior Angles:
Here are some key characteristics of interior angles:

1. Sum of Interior Angles: The sum of the measures of the interior angles of a triangle is always 180 degrees. This is known as the interior angle sum property of triangles.

2. Interior Angle Pairs: Interior angles are formed in pairs when a transversal intersects two parallel lines. The pairs of interior angles are located on the same side of the transversal and are congruent (equal in measure).

3. Non-adjacent Angles: Interior angles are non-adjacent angles, which means they do not share a common side or vertex.

4. Angle Relationships: Interior angles have specific relationships with other types of angles formed by the transversal. For example, they are supplementary to the corresponding exterior angles and alternate exterior angles.

Examples of Interior Angles:
Let's consider an example to understand interior angles better. Suppose we have two parallel lines, AB and CD, and a transversal line, EF, intersecting them.

- The angles formed between AB and EF are interior angles.
- The angles formed between CD and EF are also interior angles.
- The pairs of interior angles on the same side of EF are congruent.

Summary:
- Interior angles are the angles formed on the inside of two parallel lines by a transversal.
- They are located between the parallel lines and are non-adjacent angles.
- The sum of the measures of the interior angles of a triangle is always 180 degrees.
- Interior angles have specific relationships with other types of angles formed by the transversal, such as exterior angles and alternate exterior angles.