Contact patch is the portion of a vehicle's tire that is in actual contact with the road surface. It is most commonly used in the discussion of pneumatic tires, (i.e. pressurized tires), where the term is strictly used to describe the portion of the tires tread that touches the road surface (see image below). The term footprint is used almost synonymously.
The only connection between the road and the vehicle is the contact patch. The size and shape of the contact patch as well as the pressure distribution within the contact patch are important to the ride qualities and handling characteristics of a vehicle. Since the wear characteristics of tires is a highly competitive area between tire manufacturers, a lot of the research done concerning the contact patch is considered highly proprietary and, therefore, very little is published on the subject.
Due to the flexibility of pneumatic tires, the contact patch is different when the vehicle is in motion from when it is static. Because it is so much easier to make observations of the contact patch without the tire in motion, it is more common to conduct studies of the static contact patch.
Statically the load and tire inflation pressure affects the contact patch. The larger the load on the tire, the larger the contact patch. Conversley the larger the inflation pressure, the smaller the contact patch.
Unfortunately, these two properties are not linearly proportional to the area of the contact. Put another way, a 10% change in load or inflation pressure usually does not result in a 10% change in the contact patch. With that in mind, if you can alter the load or pressure freely, the contact patch size will always be limited by the tire geometry.