The definition of an empty leg flight is when a private chartered jet needs to reposition itself without passengers at a specific airport. This can happen when having to meet their next pick up at a different location or when returning home after a one-way charter.
An example would be a private jet that is booked from New York to Miami but is currently located in Chicago. The empty leg flight would then refer to the jet repositioning from Chicago to New York with no passengers on board.
Since the cost of this connecting flight is already included in the original fare, the unbooked leg is then sold off on a first-come, first-serve basis. Anyone who wants to fly within the jet’s already pre-determined flight path (Chicago to New York), can then book this fare for a fraction of the normal charter cost.