Also: Ferry, Überführungsflug, Transfer flight
The term originally describes any transfer of an aircraft from A to B without passengers. Typical reasons include a trip to a maintenance organisation, the delivery of a new aircraft or a change of home base.
In day-to-day charter business, ferry flight is often used interchangeably with positioning flight. From the passenger's point of view the difference is small: in both cases the aircraft flies without paying passengers.
Not every ferry flight can be sold as an empty leg. Flights to maintenance or under special permits may not be allowed to carry passengers. Whether a flight is bookable is always the operator's decision.