Addition : Addition is the mathematical operation that describes increasing a number by an amount equal to a second number. The mathematical symbol for addition is the plus sign $+.$ The term addition is also used for a generalization of this basic operation on numbers to functions, vectors and matrices.

Alternate exterior angle : When two parallel lines are crossed by a third line, alternate exterior angles are a pair of angles that are not between the two parallel lines and are on opposites sides of the crossing line. Alternate exterior angles are equal.

Alternate interior angle : When two parallel lines are crossed by a third line, alternate interior angles are a pair of angles that are between the two parallel lines and are on opposites sides of the crossing line. Alternate interior angles are equal.

Exterior angle : When looking at two parallel lines cut by a third line, exterior angles refer to angles that are not between the two parallel lines. In a polygon an exterior angle refers to the angle at a corner that lies outside the polygon.

Line : A line AB is a one-dimensional shape that includes the points A and B, all the points on the line segment in between A and B and all the points of the straight extension of the line segment beyond A and B. A line does not have an endpoint.

Parallel : Two lines in a two-dimensional plane are called parallel if they never cross.

Straight angle : A straight angle is an angle equal to $180^{\circ}.$

Third : A third either refers to the third object in an ordering or to the number $\frac{1}{3}=0.\overline{3}.$