The term "gas mileage" refers to the efficiency with which a vehicle uses gasoline or other fuel. It is typically measured in miles per gallon (mpg) and indicates how far a car can travel on one gallon of fuel. The higher the mpg rating, the better the gas mileage, meaning that the vehicle will use less fuel for every mile driven. Improving gas mileage not only saves money on fuel costs but also reduces emissions from vehicles, making it an important factor in environmental sustainability.