Count : To count means to find out the number of objects by adding one for every object.

Boundary : The boundary of a 2-dimensional shape is the 1-dimensional shape that separates the 2-dimensional shape from the rest of the 2-dimensional space.

Center : The center of a circle is the unique point in the circle, such that all points on the boundary of the circle have a constant distance equal to the radius from that point. The center of a regular polygon is the point that has equal distance from all its corners.

Chord : A chord is any line segment between two points on the boundary of a circle.

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.

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

Line segment : A line segment AB is a one-dimensional shape consisting of the points A and B and all the points on the straight connection of A and B. A and B are both endpoints of the line segment AB.

Point : A point is an element in a space. Shapes are made of sets of points.

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

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

Variable : A letter in an expression or function that represents an arbitrary number.