The horizontal line passing through the origin.
http://wiki.answers.com/Q/What_is_a_x_axis_of_a_gr...

The yaxis is the vertical line in the coordinate system. The line begins between Quadrants I and II, intersects with the horizontal xaxis at the point of origin, and then continues
http://www.ehow.com/info_8583755_doverticalaxis...

The x and y axes is where x and y = 0 and is the starting point and center for
http://www.chacha.com/question/whatisanaxison...

On a graph, the zaxis is a third dimension, the axis in threedimensional Cartesian coordinates which is usually oriented vertically.
http://www.kgbanswers.com/whatisazaxisonagr...
