Final answer:
A completely flat/horizontal line represents a slope of zero, meaning there is no change in the vertical direction as you move horizontally along the line.
Step-by-step explanation:
The slope of a line is a measure of its steepness or incline. It is defined as the ratio of the vertical change (rise) to the horizontal change (run) between two points on the line.
The question pertains to the concept of the slope of a line in mathematics. A line that is completely flat/horizontal has a slope of zero, indicating there is no vertical change as you move along the line. In more technical terms, this means there is zero rise over the run. For example, the graph of a ball's horizontal velocity that remains constant over a period of time would appear as a flat horizontal line on a velocity-time graph, representing that the velocity does not change as time progresses. This is also observed in line graphs that show different segments, and a horizontal line would represent a period where there is no change, such as an unemployment rate that remains constant over a period of time.
A slope of zero represents a horizontal line on the coordinate plane. In the context of a linear equation, a slope of zero indicates that the line is parallel to the x-axis. The slope (mm) is the ratio of the vertical change (rise) to the horizontal change (run) between two points on the line.