axis – a perpendicular reference line used on a graph

(pronounced acks-is)

To remember the meaning of axis, use the following mnemonic:

The two axes (axis) were perpendicular and formed the lines for a graph.

An axis is a fixed reference line used to locate points on a graph. In two dimensional coordinate systems, we typically use two perpendicular axes (plural of axis): The horizontal X-axis and the vertical Y-axis, which intersect at a point called the origin.

These axes form a framework that allows us to specify the position of any point using coordinates. 

In three-dimensional space, a third axis is added (the Z-axis), perpendicular to both the X and Y axes.

The Z-axis in a graph displays depth.