Angle : If two line segments (or rays) both start at a common point the opening between the two line segments is called an angle. The common point is called vertex of the angle. The size of an angle is measured in degrees.

Degree : For angles one degree is defined as the angle represented by a $\frac{1}{360}$ of the angle represented by a full circle. For measuring temperatures one can use either degrees Celsius or degrees Fahrenheit. In a polynomial the degree describes the highest exponent of $x$ that has a non-zero coefficient. For example the degree of $1+x+x^2$ is 2.

Interior angle : An interior angle at the crossing of two parallel lines by a third line means any of the angles between the two parallel lines. An interior angle of a polygon is any of the angles at any of the corners of the polygon that lies within the polygon.

Octagon : An octagon is a polygon with eight corners. The sum of the angles in an octagon is $1080^{\circ}.$

Regular polygon : A polygon is called regular if it is convex, equilateral and equiangular.

Sum : A sum is the result of an addition.

Triangle : A triangle is a polygon with three corners and three sides. You can calculate the area of a triangle by multiplying half the length of the base by the height on that base. The sum of the interior angles in a triangle is always $180^{\circ}.$