Zero Slope Definition
A slope of zero means that the line is a horizontal line. A horizontal line has slope of 0 because all of its points have the same y-coordinate. As a result, the formula used for slope evaluates to 0. (In other terms, the top part of the equation or numerator evaluates to always equal zero.)
“Slope.” Wikipedia, Wikimedia Foundation, 22 Mar. 2020, en.wikipedia.org/wiki/Slope.