Aviatomobile was a burgundy toy car, which was presumably bewitched to fly. They were sold at Fred and George Weasley's shop in Diagon Alley, Weasleys' Wizard Wheezes. It was most likely inspired by a similar, full-sized car enchanted by their father.
The "avia" in the name, may come from the Latin "avis" meaning bird, and therefore flying. "Mobile" may come from the Latin "movilis" meaning movable, which explains the mobility of the car.