Wavefront

A wavefront is a surface of constant phase function. A plane wave has planar wavefronts; variations in refractive index distort them.

The optical path length along a ray is the integral of its refractive index over geometric length. A difference $h$ in optical path length produces a phase difference $2\pi h/\lambda$, where $\lambda$ is the vacuum wavelength.

A wavefront error is the difference in optical path length from a reference wavefront, often an ideal plane or sphere. Its root mean square should specify the illuminated pupil and which modes, such as constant phase or image displacement, have been removed.