Optical pathlength

From Glossary of Meteorology

optical pathlength

(Sometimes called optical path or optical length.) The line integral of the refractive index (real part) along a ray connecting two points in an optically homogeneous medium.

Because refractive index depends on frequency, so does optical pathlength. Although optical pathlength may be shortened to optical path, there is a difference between a path and its length.
Compare optical thickness.