Source : Free On-Line Dictionary of Computing

Z-buffer
     
         An {array} used to store the maximum Z {coordinate}
        of any feature plotted at a given (X, Y) location on the
        screen, used for {hidden line removal} in a 2D rendering of a
        3D scene.  The Z axis is perpendicular to the screen with
        values increasing toward the viewer so that any point whose Z
        coordinate is less than the corresponding Z-buffer value will
        be hidden behind some feature which has already been plotted.
     
        (1997-07-18)