The coordinate plane is formed by the intersection of a vertical and a horizontal number line What is the horizontal number line called?

In the co-ordinate plane

The horizontal number line is the 'x-axis'.

The vertical number line is the y-axis.

They intersect at the 'origin' ; coordinate (0,0)

In the 3-dimensional system there is a third number line going from front to back , this is the z-axis.

All three axes intersect at the origin , and have the the coordinates ( x,y,z) = (0,0,0)