The sale of (or, in the case of overbooking, the acceptance of reservations for) more space (passenger seats) than is actually available on a flight. A practice that is used sometimes by the air carriers as an allowance for that percentage of passengers who fail, for some reason, to use the space they have reserved on a flight. In those cases where the actual number of passengers with purchased tickets exceeds the available space for the flight, the carrier is liable for denied boarding compensation to those passengers not accommodated on the flight or on comparable air transportation.