What is the difference between undefined slope and no slope?

Undefined slope is when a line is exactly on the y-axis, for reasons unknown, the slope of a line on the y-axis cannot be determined, so therefore it it "undefined."

A line with no slope is a line with a slope of zero. In order for a line to have a slope of zero, it must be directly on the x-axis, meaning it is horizontal. Horizontal lines are straight and have no slope