A few resources on public access to private land. Always check with your local planning department for zoning requirements.
Definitions 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."
The Recreational Trails Program legislative definition of a trail is:
23 U.S.C. 206:
§ 206. Recreational trails program
(a) DEFINITIONS.—In this section, the following definitions apply:
(1) MOTORIZED RECREATION.—The term ‘motorized recreation’ means off-road recreation using any motor-powered vehicle, except for a motorized wheelchair.
(2) RECREATIONAL TRAIL.—The term ‘recreational trail’ means a thoroughfare or track across land or snow, used for recreational purposes such as—
(A) pedestrian activities, including wheelchair use;
(B) skating or skateboarding;
(C) equestrian activities, including carriage driving;
(D) nonmotorized snow trail activities, including skiing;
(E) bicycling or use of other human-powered vehicles;
(F) aquatic or water activities; and
(G) motorized vehicular activities, including all-terrain vehicle riding, motorcycling, snowmobiling, use of off-road light trucks, or use of other off-road motorized vehicles.