According to the American Trails organization, the definition of a trail can be really broad or quite specific. The National Recreation Trails Program defines a trail as, "A trail is a travel way established either through construction or use and is passable by at least one or more of the following, including but not limited to: foot traffic, stock, watercraft, bicycles, in-line skates, wheelchairs, cross-country skis, off-road recreation vehicles such as motorcycles, snowmobiles, ATVs and 4-wheel drive vehicles."
If you want to learn more about the various definitions of trails, refer to the American Trails article entitled FAQ: What is the definition of a trail?