A lightpath exists in a wavelength-route optical transport network, where each fiber carries multiple wavelengths and optical nodes are capable of switching and/or wavelength-converting wavelength channels. A lightpath traverses multiple fiber links and on each link a wavelength is used to make up the lightpath. A lightpath has it source and destination nodes and is generally bi-directional; that is, there is a lightpath on one direction, and another lightpath on the other direction.

Depending on the wavelength conversion capability in optical networks, if a lightpath uses the same wavelength on all the traversed links, then it is termed wavelength path (WP); it features of the wavelength continuity on the whole path; otherwise, the lightpath is termed virtual lightpath as the it uses different wavelengths on different path segment.