Also: Time slot, Airport slot, ATC slot
There are two kinds. An airport slot is permission to take off or land at a busy, coordinated airport at a specific time. An air traffic control slot sets the departure time when airspace along the planned route is congested.
Major airports such as Frankfurt, Munich and Zurich are coordinated, and private jets need a slot there too. During trade fairs, sporting events or the winter season, slots can become scarce.
For passengers, this means departure times cannot always be moved at will. Short-notice changes depend on whether a new slot is available.