The measure of the tilt of a line, the rise over run or how much the line moves up for every movement to the right.
For a straight line in the plane, the slope is the tangent of the angle it forms with the positive X axis.
For a curve the slope is the slope of the tangent line.
If the slope is constant a line is straight.
A line joining two points
The slope is given by:
m = slope
x1,y1 = cartesian coordinates of point 1
x2,y2 = cartesian coordinates of point 2