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.

Right angle : A right angle is an angle equal to $90^{\circ}.$

Square : A square is a quadrilateral with four right angles and four equal length sides. A square is a special case of a rectangle and a special case of a rhombus.

Sum : A sum is the result of an addition.

Equilateral : A polygon that has side lengths that are all equal is called equilateral polygon.

Equilateral triangle : An equilateral triangle is a triangle with three sides of equal length. An equilateral triangle is also equiangular and has interior angles of $60^{\circ}.$

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.

Isosceles triangle : An isosceles triangle is a triangle in which two side length are the same. The two angles at which these two sides meet the third side are the same.

Length : Length is the attribute of a one-dimensional shape that can be measured with a measuring tape.

Obtuse angle : An obtuse angle is an angle bigger then $90^{\circ}.$

Obtuse triangle : An obtuse triangle is a triangle that has one obtuse angle.

Right triangle : A right triangle is a triangle that has a right angle. The sides that form the right angle are called legs, the third side is called hypotenuse. The area of a right triangle is half the product of the lengths of the two legs. The Pythagorean theorem is an important theorem for right triangles that allows to calculate the third side of a right triangle given the other two.

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

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 $180^{\circ}.$

Decimal : A decimal is a number represented by digits in front and after a decimal point. In front of the decimal point the digits are the ones digit, tens digit, hundreds digit and so on. After the decimal point there are the tenths digit, hundredths digit, thousandths digit and so on. For example the decimal 1.3 can be written as $1\frac{3}{10}.$

Even : Even can refer either to an even function or an even number.

Fraction : A fraction is a number that can be written as $\frac{a}{b}$ with an integer $a$ and a natural numbers $b.$ A fraction $\frac{a}{b}$ is the result of the division $a\div b.$ Examples of fractions are $\frac{1}{2}, -\frac{2}{3}$ and $\frac{3}{2}.$

Rational number : A rational number is a number of the form $\frac{a}{b}$ with integers $a, b.$

Square root of 2 : The square root of 2 also written as $\sqrt{2}$ is the positive number such that $(\sqrt{2})^2=\sqrt{2}\cdot \sqrt{2}=2.$ $\sqrt{2}$ is an irrational number and rounded to the nearest hundredths it is equal to 1.41.

Whole number : A whole number is any of the numbers $0,1,2,\ldots.$