The term is applied for the axis of a graph. The horizontal axis is
the x-axis, the vertical y-axis and the z-axis is a possible third axis
used for three-dimensional graphing. Each axis possesses a unit and sizing
markers. Axis properties can be customized in the preferences dialogs:
In general, all known units can be used with any axis in a data set.
However, in most cases the x-axis is used as data point axis, whereas
the y- and/or z-axis are used as intensity axes. If any unknown unit is
provided by an axis, the arbitrary unit is applied automatically.