What Is Slope?
Slope measures how much a line rises or falls vertically for every unit it moves horizontally, calculated as the change in y divided by the change in x between two points. A positive slope rises left to right, a negative slope falls, a slope of 0 is flat, and an undefined slope is a vertical line.
Example
The line through (1, 2) and (4, 11) has a slope of (11−2)/(4−1) = 9/3 = 3 — for every 1 unit the line moves right, it rises 3 units.