The intuitive path that is created by walkers/hikers or drivers vs a marked trail or road. Once established it can become an alternative or preferred route. Typically it is a short cut on a marked trail or road but it can also be a well-worn ribbon of exposed soil or rut in an open space where there are few trails to connect desired areas.
The desire line became so popular that the marked trail became overgrown and hard to follow from lack of use. However, over time the desire lines became so eroded they were unusable and abandoned in favor of the switch back trails.