Answer:
The "slope" of a vertical line. A vertical line has undefined slope because all points on the line have the same x-coordinate. As a result the formula used for slope has a denominator of 0, which makes the slope undefined..
Explanation:
The slope of a vertical line does not exist! We can't divide by zero, which is of course why this slope value is "undefined". This relationship is always true: a vertical line will have no slope, and "the slope is undefined" or "the line has no slope" means that the line is vertical.