It 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 tire’s tread that touches the road surface. The term footprint is used almost synonymously.
"Did you see that car have no contact patch?"
by Known-Unknown December 20, 2009