Fender is the American English term for the part of an automobile, motorcycle or other vehicle body that frames a wheel well (the fender underside). Its primary purpose is to prevent sand, mud, rocks, liquids, and other road spray from being thrown into the air by the rotating tire. Fenders are typically rigid and can be damaged by contact with the road surface.

Sticky materials, such as mud, may adhere to the smooth outer tire surface, while smooth loose objects, such as stones, can become temporarily embedded in the tread grooves as the tire rolls over the ground. These materials can be ejected from the surface of the tire at high velocity as the tire imparts kinetic energy to the attached objects. For a vehicle moving forward, the top of the tire is rotating upward and forward, and can throw objects into the air at other vehicles or pedestrians in front of the vehicle.

In British English, the fender is called the wing. (This may refer to either the front or rear fenders. However, in modern unibody vehicles, rear fenders may also be called quarter panels.) The equivalent component of a bicycle or motorcycle, or the "cycle wing" style of wing fitted to vintage cars, or over tires on lorries which is not integral with the bodywork, is called a mudguard in Britain, as it guards other road users – and in the case of a bicycle or motorcycle, the rider as well – from mud, and spray, thrown up by the wheels.

In modern Indian and Sri Lankan English usage, the wing is called a mudguard. However, the term mudguard appears to have been in use in the U.S. at one point. The American author E.B. White, in his October 1940 Harper's essay "Motor Cars", refers to "...mudguards, or 'fenders' as the younger generation calls them."

In the United States, a minor car accident is often called a "fender bender".