Question

what is slope........in your language....

Answer 1

Anything that has a direction can have a slope. In order to define that, you need to choose a coordinate system, and you have to designate one line or plane in your coordinate system as the “horizontal” reference. 
slope = [ f(x1) = f(x0) ] / (x1 - x0).

Answer 2

If a line passes through two points (x1,y1) and (y1,y2) then slope of the line is given by m= y2-y1/x2-x1 .