Axis is the orientation of the [[cylinder]] portion of your lens [prescription], used to compensate for [[astigmatism]]. You don't have an axis value if you don't have [[astigmatism]]. From the lens wearer's perspective, 0 degrees of axis is horizontal, and increasing values move clockwise around the eye. The axis of cylinder is symmetrical, so it only goes from 0 to 180 degrees.
Axis of astigmatism can be measured with a [[stenopaeic slit]] or [[JCC lens]] but the use of these tools is complex and easily misunderstood, it's usually best to get your axis off of your most recent prescription.