The path difference of two waves, which travel to the observation point (e.g. to the point where a bright interference fringe can be seen). The path difference thus indicates by how many wavelengths a wave leads or lags another wave.
For constructive interference, the path difference takes the following values: \[ \Delta s ~=~ 0,~ \lambda,~ 2 \, \lambda,~ 3 \, \lambda, ~... \]
Integer multiple of the wavelength takes the following values in the case of constructive interference: \( m = 0, 1, 2, ... \). This integer indicates the factor by how much one wave precedes the other.
Wavelength of the two waves under consideration. So the wavelength of the light used.