• A Segment is the operation of a flight from a commercial perspective. It features a single flight designator (flight number) and takes passengers from the point where they first board an aircraft to the point where they leave the flight, either completing the journey or boarding another flight with a different flight number.
• A Leg is an operational trip of an aircraft from one airport to another. Short flights are usually 1 leg. Long-haul may have 2 or more legs. The rough rule of thumb is that segments are what you buy, legs are what you fly.
• Passenger Boarded (PBs) - is a segment flown by a passenger.
• Passenger Booked is a segment reserved by a passenger and confirmed by the airline. For most airlines the number of passengers booked is greater than the number of PBs due to no-shows.
• PAX - for airlines it is an abbreviation for a Passenger. In hotels it refers to a Person or occupant.
• PNR - Passenger Name Record. It is a virtual container which can include a selection of flights, car rental bookings, hotel bookings, train journeys and other parts related to a passenger journey. It also contains identification and contact information for the passenger(s). Most PNRs also contain information about the fare calculation and the tickets issued (if any). https://en.wikipedia.org/wiki/Passenger_name_record
• Booking - This can have different meanings. It can be the same as a PNR or a passenger booked on a segment. It does not imply that the passenger’s journey has been ticketed.
• Ticket - is an accountable document issued by traditional airlines. In most cases it represents the value of the fare paid and any applicable taxes and charges. Tickets are used to exchange information between carriers in a traditional interline or codeshare relationship and to establish the right to fly in airport departure control systems. Since 2008 all air tickets are electronic and reside in ticketing servers rather than being produced in hard copy.
• Coupons - Each ticket contains one or more coupons. Each coupon corresponds to a booked segment. The coupon eventually corresponds with a boarding pass.
• Ticketless In ticketless systems, the PNR also serves the main functions of a ticket. That does mean, however that the PNR may go through status changes depending on when it was created and paid for, and other factors.

Simple Examples

• A one-way flight without a stop (eg. LHR-CDG) for one person is
  1 Segment
  1 Leg
  1 PB
  1 PAX
  1 PNR
  1 Booking
  1 Ticket containing 1 Coupon
  
• A return flight for 1 person on this route LHR - CDG > CDG - LHR booked on 1 ticket.
  2 Segments
  2 PBs
  1 PAX
  1 PNR
  1 Booking
  1 Ticket containing 2 Coupons
  
  
• A return flight for 2 people on this route LHR - CDG > CDG - LHR booked on 1 ticket.
  4 Segments
  4 PBs
  2 PAX
  1 PNR
  1 Booking
  2 Tickets each with 2 coupons
  

• It is not always obvious how many segments, and thus PBs are counted in a journey.
• Example 1
  British Airways flight BA0015 is London to Sydney. It is one flight number all the way but it does stop to refuel in Singapore. So a passenger travelling LHR-SYD on BA0015 is one segment. There is one ticket with a single coupon.
  However if you fly LHR-SIN on BA11 and then SIN-SYD on BA0015, this accounts for two segments each with one leg. In this case there is one ticket with two coupons.
  
  
• Example 2
  A Southwest Airlines customer buys a ticket HOU - MCI, but the plane does HOU - DAL - MCI. This is one segment but with two legs. It is one PB. Southwest manages the through-passengers by counting the people in the cabin at the stopping point. (they call it the through count).

As airlines gradually change to systems based on Offer-Order-Settle-Deliver some of the existing artefacts will disappear and be replaced by new ones. The most significant new definitions are:

• An Offer is a proposition made by an airline to a specific Customer to provide a package of Services which may include flight segments and other products such as baggage, seat selection, WiFi, ground transport, hotel accommodation etc.
• An Offer Item is a component of an Offer that contains one or more Services that are indivisible. That is they must be accepted by the Customer as a package, and at the quoted price, or not at all.
• A Service is the product that will be delivered to the Passenger such as the right to fly on a specific segment, the ability to check a bag or a WiFi package. Each Service has an associated status that shows where it is in the progression from booking to payment to delivery.
• A Customer is the entity that requests Offers from the airline. It may be an individual or a company.
• A Passenger is the person who actually uses the Services. The customer and the passenger may or may not be the same individual.
• An Order is created when a Customer accepts one or more Offer Items. It replaces both the PNR and the Ticket. Each accepted Offer Item results in the creation of an Order Item. Order Items are indivisible in the event that the customer wishes to partially change or cancel Services. Service status is tracked within the Order.