The Y-axis is the vertical line on a graph or chart that is used to represent and measure values. It typically runs from the bottom to the top of the graph and is perpendicular to the X-axis, which runs horizontally. In most cases, the Y-axis displays the dependent variable, showing how it changes in response to the independent variable plotted along the X-axis. The Y-axis is commonly used in mathematics, science, engineering, and data visualization to help interpret and compare data points.