An empty leg is the repositioning portion of a private jet charter flight. A private jet empty leg comes about when an aircraft needs to fly without any passengers, in order to reposition itself to the starting destination for its next customer. Or to fly back to its home base. As the aircraft needs to be moved anyway the operator will often be prepared to sell the empty part of the flight at a significant discount, compared to the full charter price.
If you are prepared to be flexible, an empty leg flight can be a great way of accessing private jet travel, at lower prices. An empty leg can be combined with commercial airline flights or worked into a private charter schedule, to give a more cost-effective price for a private charter trip.