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

Acute angle : An acute angle is an angle that is smaller than $90^{\circ}.$

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.

Counterexample : A counterexample to a statement of the form 'all objects of type x have property y' is an object of type x that does not have property y. A counterexample to a statement shows that the statement is not true in general. For example the number 3 is a counterexample to the statement that all numbers are even.

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 $90^{\circ}.$

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

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.

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 $180^{\circ}.$

Coordinate system : A coordinate system is a system to describe and visualize points in space (most commonly a two-dimensional or three-dimensional space). Every point is described by a number of coordinates equal to the dimension of the space.

Function : A function is a mapping in which every element in one set is mapped to exactly one element of a second set. Most often the mapping is described using a rule. For example the function $f(x)=x+1$ maps 2 to 3 and -1 to 0.

Graph : The graph of a function is the set of points $\{(x,f(x))\}$ that can be blotted in a coordinate system.

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

X-axis : The horizontal axis in a two-dimensional coordinate system.

X-coordinate : The first coordinate in a two-dimensional coordinate system.

Y-axis : The vertical axis in a two-dimensional coordinate system.

Y-coordinate : The second coordinate in a two-dimensional coordinate system.