Official term states that it's a trail that is just wide enough for a single rider to fit at one time. However, there are different widths it seems. "Narrow Singletrack" I think is generally regarded as anything that's 6" wide or less. then of course there's the concept of twisty singletrack... all these descriptions are highly subjective. Other types of labels for trail width and conditions include doubletrack, jeep trails and fireroads.