The slope of a line measures its steepness and is defined as m = tan(theta), where theta is the angle the line makes with the positive x-axis measured counter-clockwise. For a line through (x1, y1) and (x2, y2), m = $\frac{y2 - y1}{(x2 - x1)}$. The slope is undefined for vertical lines (x = constant). A horizontal line has slope 0. A line making 45 degrees with the x-axis has slope 1. Negative slopes indicate lines falling from left to right.