Wavefront Error
Wavefront error is the measure of a lens or mirror's inability to bring all the incoming light rays to a common focus. It is measured from the Peak to the Valley. The Peak is the location where the ray of light that is furthest away from the lens or mirror crosses the lens or mirror's axis of symmetry. The Valley is the location where the ray of light that is closest to the lens or mirror crosses the lens or mirror's axis of symmetry.  In the above diagram the lens is towards the left, and the lens' axis of symmetry is shown in blue. Reference lines are red.