What is the definition of "y axis"?

The y axis is typically the vertical axis in a two-dimensional coordinate system. The y coordinate is measured along this axis. It is crossed by, or perpendicular to, the horizontal x axis.

The point at which the two axes join is the {0,0} point, with the first zero being the x coordinate and the second the y coordinate. In a three-dimensional coordinate system, the y axis is one of three axes, with the other two being the x axis and the z axis. Another name for the y axis is the axis of ordinates. The coordinate system that uses these axes is known as the Cartesian coordinate system.