The floorpan is a large sheet metal

Sheet metal

Sheet metal is simply metal formed into thin and flat pieces. It is one of the fundamental forms used in metalworking, and can be cut and bent into a variety of different shapes. Countless everyday objects are constructed of the material...

 stamping

Stamping (metalworking)

Stamping includes a variety of sheet-metal forming manufacturing processes, such as punching using a machine press or stamping press, blanking, embossing, bending, flanging, and coining. This could be a single stage operation where every stroke of the press produce the desired form on the sheet...

 that often incorporates several smaller welded

Welding

Welding is a fabrication or sculptural process that joins materials, usually metals or thermoplastics, by causing coalescence. This is often done by melting the workpieces and adding a filler material to form a pool of molten material that cools to become a strong joint, with pressure sometimes...

 stampings to form the floor of a large vehicle

Vehicle

A vehicle is a device that is designed or used to transport people or cargo. Most often vehicles are manufactured, such as bicycles, cars, motorcycles, trains, ships, boats, and aircraft....

 and the position of its external and structural panels.
In the case of monocoque

Monocoque

Monocoque is a construction technique that supports structural load by using an object's external skin, as opposed to using an internal frame or truss that is then covered with a non-load-bearing skin or coachwork...

 designs, the floorpan is the most important metal part establishing the chassis

Chassis

A chassis consists of an internal framework that supports a man-made object. It is analogous to an animal's skeleton. An example of a chassis is the underpart of a motor vehicle, consisting of the frame with the wheels and machinery.- Vehicles :In the case of vehicles, the term chassis means the...

, body, and thus the car’s size. It serves as the foundation of most of the structural and mechanical components of a unibody automobile

Automobile

An automobile, autocar, motor car or car is a wheeled motor vehicle used for transporting passengers, which also carries its own engine or motor...

 to which the powertrain

Powertrain

In a motor vehicle, the term powertrain or powerplant refers to the group of components that generate power and deliver it to the road surface, water, or air. This includes the engine, transmission, drive shafts, differentials, and the final drive...

, suspension system, and other parts are attached.

The term is also applied to the smaller stamped panels that form the floors inside a vehicle as well as the bottom of the trunk

Trunk (automobile)

The trunk or boot of an automobile or car is the vehicle's main storage, luggage, or cargo compartment. Trunk is used in North American English and Jamaican English; boot is used elsewhere in the English speaking world. Trunk is also primarily used in many non-English speaking regions, such as...

.