Angle- Two rays that share the same endpoint form an angle. The point where the rays intersect is called the vertex of the angle. The two rays are called the sides of the angle.

Acute Angles- An acute angle is an angle measuring between 0 and 90 degrees.

Obtuse Angles- An obtuse angle is an angle measuring between 90 and 180 degrees.

Right Angles- A right angle is an angle measuring 90 degrees.

Complementary Angles- Two angles are called complementary angles if the sum of their degree measurements equals 90 degrees.

Supplementary Angles- Two angles are called supplementary angles if the sum of their degree measurements equals 180 degrees.

Vertical Angles- For any two lines that meet, such as in the diagram below, angle AEB and angle DEC are called vertical angles. Vertical angles have the same degree measurement. Angle BEC and angle AED are also vertical angles.

Alternate Interior Angles- For any pair of parallel lines 1 and 2, that are both intersected by a third line, such as line 3 in the diagram below, angle A and angle D are called alternate interior angles. Alternate interior angles have the same degree measurement. Angle B and angle C are also alternate interior angles.

Alternate Exterior Angles- For any pair of parallel lines 1 and 2, that are both intersected by a third line, such as line 3 in the diagram below, angle A and angle D are called alternate exterior angles. Alternate exterior angles have the same degree measurement. Angle B and angle C are also alternate exterior angles.

Corresponding Angles- For any pair of parallel lines 1 and 2, that are both intersected by a third line, such as line 3 in the diagram below, angle A and angle C are called corresponding angles. Corresponding angles have the same degree measurement. Angle B and angle D are also corresponding angles.

Angle Bisector- An angle bisector is a ray that divides an angle into two equal angles.

Perpendicular Lines- Two lines that meet at a right angle are perpendicular.