Section 20. In sections twenty-one to thirty-five, inclusive, the following words shall, unless the context requires otherwise, have the following meanings:—

''All-terrain vehicle'', a motorized recreational vehicle designed or modified for travel on 4 low pressure tires and having a seat designed to be straddled by the operator and handlebars for steering control.

''Directly supervised'', the supervising adult shall be sufficiently close to the operator at all times that the operator's vehicle is in operation, such that a reasonable person acting as supervisor under the totality of the circumstances including, but not limited to, vehicle and ambient noise, the landscape, topography and geography of the location, and the operator's wearing of protective headgear, would reasonably believe that he is maintaining visual contact and verbal communication with the operator.

''Law enforcement officer'', the director, a deputy director of enforcement, a chief of enforcement, a deputy chief of enforcement, an environmental police officer and a deputy environmental police officer of the office of environmental law enforcement, the registrar or an authorized agent thereof, a police officer, a member of the state police, a city or town police officer or an employee of the commonwealth having police powers on public lands.

''Operate'', to ride in or on and control the operation of a snow vehicle or a recreation vehicle.

''Owner'', a person, other than a lien holder, having the property or title to a snow vehicle or to a recreation vehicle entitled to the use or possession thereof.

''Recreation utility vehicle'' or ''utility vehicle'', a motorized flotation tire vehicle with not less than 4 and not more than 6 low pressure tires that is limited in engine displacement to less than 1,500 cubic centimeters and in total dry weight to not more than 1,800 pounds and that has a seat that is of bench design, not intended to be straddled by the operator, and a steering wheel for control.

''Recreation vehicle'' or ''off-highway vehicle'', any motor vehicle designed or modified for use over unimproved terrain for recreation or pleasure while not being operated on a public way as defined in chapter 90 including, but not limited to, all-terrain vehicles, off-highway motorcycles, dirt bikes, recreation utility vehicles and all registered motor vehicles while not being operated on a public way as defined in said chapter 90; provided, however, that recreation vehicles and off-highway vehicles operated exclusively for agricultural, forestry, lumbering or construction purposes shall not be subject to this chapter and it shall be an affirmative defense that such vehicle was being operated for such purposes at the time of an alleged violation of this chapter.

''Snow vehicle'', a motor vehicle designed to travel over ice or snow, having a curb weight of not more than 453 kilograms or 1,000 pounds, driven by track or tracks in contact with the snow or ice and steered by a ski or skis in contact with the snow or ice.