An all-terrain vehicle (ATV), also known as a light utility vehicle (LUV), a quad bike, or simply a quad, as defined by the American National Standards Institute (ANSI), is a vehicle that travels on low-pressure tires, has a seat that is straddled by the operator, and has handlebars. As the name implies, it is designed to handle a wider variety of terrain than most other vehicles. Although it is a street-legal vehicle in some countries, it is not street-legal within most states, territories and provinces of Australia, the United States or Canada.

By the current ANSI definition, ATVs are intended for use by a single operator, but some companies have developed ATVs intended for use by the operator and one passenger. These ATVs are referred to as tandem ATVs.

The rider sits on and operates these vehicles like a motorcycle, but the extra wheels give more stability at slower speeds. Although most are equipped with three or four wheels, six-wheel models exist and existed historically for specialized applications. Multiple-user analogues with side-by-side seating are called utility terrain vehicles (UTVs) or side-by-sides to distinguish the classes of vehicle. Both classes tend to have similar powertrain parts. Engine sizes of ATVs currently for sale in the United States (as of 2008 products) range from 49 to 1,000 cc (3.0 to 61 cu in).