Gradient (Slope) of a Straight Line
The Gradient (also called Slope) of a straight line shows how steep a straight line is.
Calculate
The method to calculate the Gradient is:
Gradient = 

Examples:
So the Gradient is equal to 1 
Gradient = 

= 2  
The line is steeper, and so the Gradient is larger.
Gradient = 

= 0.6  
The line is less steep, and so the Gradient is smaller.
Positive or Negative?
Important:
 Starting from the left end of the line and going across to the right is positive
(but going across to the left is negative).  Up is positive, and down is negative
Gradient = 

= −2  
That line goes down as you move along, so it has a negative Gradient.
Straight Across
Gradient = 

= 0  
A line that goes straight across (Horizontal) has a Gradient of zero.
Straight Up and Down
Gradient = 

= undefined  
That last one is a bit tricky ... you can't divide by zero,
so a "straight up and down" (Vertical) line's Gradient is "undefined".
Rise and Run
Sometimes the horizontal change is called "run", and the vertical change is called "rise" or "fall":
They are just different words, none of the calculations change.