Two dimensions are normally measured using an 'X' and a 'Y' axis that are at right angles to one another. For example a standard graph will have a horizontal x axis and a vertical y axis.
To describe three dimensions, the 'z' axis is used. This is at right angles to both the x and y axis. For example, if you imagine your computer screen having a horizontal x axis and a vertical y axis, the z axis is an imaginary axis going into and out of the screen. This means the z-axis is often used to describe the relative depth of something.
The z axis is widely used in computer graphics and gaming software to describe objects that are in-front-of or behind one another. Negative valus tend to go towards the back and positive z axis are towards the front.
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Click on this link: Z-Axis