What an empty leg actually is

An empty leg is a one-way sector an aircraft has to fly without passengers: home to base after a drop-off, or out to collect the next trip. The operator is paying for fuel, crew and landing fees either way. Selling the cabin at a discount recovers some of that cost.

The route and window are set by the original charter. That is the trade: a genuine saving, in return for less flexibility. If the originating trip moves, the empty leg can move or disappear with it.

This site lists empty legs only. Full-price on-demand charter is a different product, and mixing the two is how most “deal” pages become confusing.