Definition

1. The Cartesian coordinate system is used to describe the position of points and lines on a plane.
2. The rectangular or Cartesian plane is formed by two perpendicular lines which intersect at the origin which is denoted by the letter "O".
3. The horizontal axis is called the x-axis while the vertical axis is called the y-axis. 
4. Points on the graph are represented using coordinates. Its position can be determined by its distance from each of the axes. 
5. In general, the position of any point P on the plane can be located by an ordered pair (a, b). Here, a is the x-coordinate of P while b is the y-coordinate of P. 

Definition and Gradient

1. A linear graph is one where the points can be joined to give a straight line. 
2. The equation of a linear graph is of the form
y = mx + c, where m and c are constants. 

Gradient of Linear Graphs
1. A gradient of a straight line is the measure of its steepness or slope.
2. The gradient of a line is the ratio of the vertical change (the rise) to the horizontal change (the run).
- The greater the gradient, the steeper the slope.
- The gradients of upwards sloping graphs are always positive and vice versa. 
3. The gradient of a horizontal line is 0, and the gradient of a vertical line is undefined.