14 C.F.R. § 250.1
Airport means the airport at which the direct or connecting flight, on which the passenger holds confirmed reserved space, is planned to arrive or some other airport serving the same metropolitan area, provided that transportation to the other airport is accepted (i.e., used) by the passenger.
Alternate transportation means air transportation with a confirmed reservation at no additional charge, operated by a carrier as defined below, or other transportation accepted and used by the passenger in the case of denied boarding.
Carrier means:
Class of service means seating in the same cabin class such as First, Business, or Economy class, or in the same seating zone if the carrier has more than one seating product in the same cabin such as Economy and Premium Economy class.
Confirmed reserved space means space on a specific date and on a specific flight and class of service of a carrier which has been requested by a passenger, including a passenger with a "zero fare ticket," and which the carrier or its agent has verified, by appropriate notation on the ticket or in any other manner provided therefore by the carrier, as being reserved for the accommodation of the passenger.
Fare means the price paid for air transportation including all mandatory taxes and fees. It does not include ancillary fees for optional services.
Stopover means a deliberate interruption of a journey by the passenger, scheduled to exceed 4 hours, at a point between the place of departure and the final destination.
Zero fare ticket means a ticket acquired without a substantial monetary payment such as by using frequent flyer miles or vouchers, or a consolidator ticket obtained after a monetary payment that does not show a fare amount on the ticket. A zero fare ticket does not include free or reduced rate air transportation provided to airline employees and guests.
14 C.F.R. §250.1