Customs pre-clearance
Pre-clearance is US customs and immigration inspection carried out before departure, at airports such as Dublin, Shannon, Aruba, Nassau, Bermuda, Toronto, Vancouver and Abu Dhabi, so the aircraft arrives in the United States as a domestic flight and passengers walk straight out. For business aviation, Shannon is the classic stop for transatlantic flights, and it lets the aircraft land at US airports with no customs facility of their own. In Europe, the equivalent concept is a first point of entry into the Schengen area; once cleared in, say, Nice, you can continue to Florence without further formalities. Pre-clearance cuts arrival time but adds a stop and a fee.