Area : The amount of unit squares that is needed to cover a 2-dimensional shape.

Cube : A cube is a three-dimensional shape with 6 faces that are all squares. A cube has 8 corners and 12 edges.

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.

Shape : A shape is a set of points in space.

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 $180^{\circ}.$

Two-dimensional shape : A two-dimensional shape is a shape that has an area but not a volume.

Volume : Volume refers to the size of a three-dimensional objects measured by how many unit cubes it would take to fill the shape.

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.

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

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

Factor : If a natural number is divisible by a second natural number, that second natural number is called a factor of the first natural number. For example the factors of 12 are 1,2,3,4,6 and 12.

Similar triangle : A similar triangle is a triangle with the same shape but not necessarily the same size. Ratios of corresponding side lengths and corresponding angles are the same in similar triangle. This fact is used to be able to uniquely define the trigonometric functions using ratios of side lengths in right triangles. Two triangles are similar if they share two angles (and therefore all three) or if all 3 ratios of the lengths of corresponding sides in the two triangles are the same. Two triangles are also similar if two ratios of lengths of corresponding sides in the two triangles are the same and the angle between the two sides in the two triangles is the same.

Similarity : Two shapes are called congruent if they have the same shape and size. Corresponding angles are the same in similar shapes and the ratio of corresponding side length is constant.

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 $180^{\circ}.$