The document defines slope as the ratio of the "rise" over the "run" between two points on a line. Specifically, if the points are (x1, y1) and (x2, y2), then the slope m is equal to (y2 - y1) / (x2 - x1). It also discusses how to calculate the slope of a line given two points, and how the slope indicates whether a line rises or falls from left to right. Lines between the first and third quadrants have positive slopes, while lines between the second and fourth quadrants have negative slopes.