assuming you're speaking of a horizontal line on a graph: It is because the line moves neither up or down. slope is the steepness of a line and a horizontal line isn't steep at all, it has no steepness.
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Yes, it is true; slope zero is no slope.
A horizontal line has a slope of zero. The equation of a horizontal line is y = a.
Horizontal lines always have a slope of zero. (i.e completely flat, level surfaces have a slope of zero). However a line does not have to have a slope of zero in order to be a line.
a slope of zero. horizontal is undefined
zero is horizontal, undefined is vertical