"A close look at any city park or green will typically reveal footprints that break away from paved walks, trails that countless pedestrians have worn into the grass. Such a trail is a desire path: the route people have chosen to take across an open place, making a human pattern upon the landscape."
--Lan Samantha Chang in Home Ground: Language for an American Landscape, edited by Barry Lopez and Debra Gwartney.
The terms 'desire path', 'desire line' and 'social trail' are used in a semi-formal fashion in the realm of landscape architecture to refer to any path that is worn into the landscape through frequent use, but was not planned for. For obvious reasons, desire paths are almost exclusively shortcuts, as pedestrians head for their destination by the most direct route possible.
Landscape designers, and quite frankly, most humans, like looking at curving paths that are associated with natural forms and peaceful walks. Failing that, more utilitarian spaces are designed with no-nonsense paths connecting buildings in a grid-like pattern. Pedestrians with a goal in mind, however, tend to prefer a web, with direct lines between points of interest. Lawns and flower gardens that get in the way are likely to be trampled.
The existence of a desire path might be seen as evidence of failure on the part of the landscape designer -- or if you prefer your aesthetics unbound by considerations of utility, you might view desire paths as evidence of human bloodymindedness. Whatever your viewpoint, a desire path is clear evidence that the visitors to an area feel that the designed walkways do not adequately meet their needs.
"Pave where the path is. An oft-told story (perhaps apocryphal) tells how a brilliantly lazy college planner built a new campus with no paths built in at all. She waited for the first winter and photographed where people made paths in the snow between the buildings. Next spring, that's where the paving went. Some design is better if it's postponed."
-- How Buildings Learn by Stewart Brand (1994)
Unfortunately, there are a host of reasons why this tale might not be true; local codes often require that all of the sidewalks be complete before occupancy, and other rules determine the maximum slope of walkways and require adequate drainage. However, many planners do look as desire paths in their local area and in similar environments before choosing where to place their walkways, and predicting where desire paths will be likely to appear in a place like a collage campus is generally not too difficult.
Michigan State University has walkways that attempt to obviate the formation of desire paths. Whatever their process for coming up with these paths, it appears to be quite effective, and looks quite striking from above:
North Campus, West;
North Campus, East;