path difference (wave)

The "path difference" actually refers to "optical path difference", which is given by n.t, where n is the refractive index of the medium and t is th thickness of that medium.

The reason why optical path difference, instead of geometrical path difference, is used is that the the wave travels slower in a medium than in air and hence the wavelength of the wave is shortened. Such effect needs to be taken into account when considering the phase difference between two waves.

When the waves are travelling in air, n = 1. The optical path difference thus is the same as geometrical path difference.