👉 In mathematics, a "z-axis" is a coordinate axis in a two-dimensional plane. It represents the horizontal direction on a Cartesian coordinate system and is perpendicular to the vertical direction. The z-axis is often used for representing spatial dimensions or for plotting coordinates. For example, if we have a 2D plot with x and y axes, the z-axis could be represented by the line that runs through the origin (0,0) and points up, as shown in this image: 