Centimeter : A centimeter ($cm$) is $\frac{1}{100}$ of a meter.

Circle : A circle is a two-dimensional shape. The boundary of a circle is a set of points that has the same distance (the radius) from the center of the circle.

Circumference : The perimeter of a circle is called circumference. The length of the circumference $C$ can be calculated as $C=2\pi r=\pi d,$ where $r$ is the radius and $d$ the diameter of the circle. This means that the circumference of a circle with diameter 1 $cm$ (radius 0.5 $cm$) is about 3.14 $cm.$

Diameter : The diameter of a circle is the length of a line segment from the boundary of a circle through the center of the circle to the boundary on the other side of the circle.

Pi : Pi or $\pi$ is an irrational number approximately equal to 3.14 that is defined as the quotient of the circumference and the diameter of a circle.

Radius : In a circle or sphere the radius is the distance between the center and the boundary.

Half : A Half is the number equal to $\frac{1}{2}=0.5.$

Impossible : An event is impossible if it has a probability of 0.

Outcome : A result of a random experiment.

Probability : A probability is a number between 0 and 1 that describes how likely an outcome is. For discrete events that are equally likely the probability can be calculated by dividing the number of favorable outcomes by the number of possible outcomes.

Unlikely : An event in a probability experiment in unlikely if the probability of it happening is low.

Absolute value : The absolute value $|x|$ of a number $x$ is defined as the value of $x$ ignoring any potential minus sign. For example $|-2|=2$ and $|2|=2.$ In other words $|x|=x$ for $x\geq 0$ and $|x|=-x$ for $x\lt 0.$

Equation : An equation is a mathematical statement in which two expressions are written with an equal sign in between. A solution of an equation is a set of variables that makes the statement a true statement.

Equivalence : Equivalent fractions are fractions that represent the same number. Equivalent equations or inequalities are equations or inequalities that have the same set of solutions.

Expression : An expression is a succession of constants, variables and mathematical operations that if evaluated for specific values of the variables represents a number.

Follow from : A second equation or inequality follows from an initial equation or inequality if all solutions of the initial equation are also solutions from the second equation or inequality. It does not necessarily have to be the case that all solutions of the second equation or inequality have to by solutions of the first inequality. For example $x^2=1$ follows from $x=1$, but $x^2=1$ is not equivalent to $x=1$ as -1 is a solution of $x^2=1$ but not a solution of $x=1.$

Negative : A number is negative if it is $\lt 0.$

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.

Squared : $x$ squared refers to the number $x^2=x\cdot x.$ For example 3 squared equals 9.