carrier – interchangeable with “airline” most of the time 

carrier code – a two-digit (or in rare cases, 3-digit) code indicating the airline, such as UA=United, LH=Lufthansa, EK=Emirates, TG=Thai Airways, F9=Frontier, FL=AirTran, B6=Jet Blue, AF=Air France, BA=British Airways, etc.

charter carrier – a carrier that does not have scheduled service and individually ticketed seats, but rather sells entire flights, on an ad hoc basis, via travel promoters, etc. 

LCC – low-cost carrier. The term is somewhat malleable but usually, LCCs do not sell through travel agents. LCCs may have a single cabin class, fewer amenities, may avoid unionized labor, and/or may use a single equipment type; rely more on secondary airports. Examples of LCCs are Southwest, JetBlue, and Ryan Air. 

legacy carrier – a larger and long-established airline such as Delta, United, American. 

code-share a single flight can have multiple flight numbers. The operating airline and the various marketing airlines all have their own flight numbers. 

marketing airline – airlines often sell tickets under one brand that are really for another airline’s services. The seller is known as the “marketing airline”. It’s the airline whose flight number and 2-letter code is on your ticket. 

dominant marketing airline  for all the segments a passenger flies on a trip, what was the most common marketing airline. For example, if a passenger flies from SYR-SYD as follows: SYR-LGA on American, LGA-LAX on American, and LAX-SYD on Qantas, then Qantas (QF) is the dominant airline. Normally, each airline’s individual segment mileages are added together when determining the dominant carrier. 

operating airline – the airline that appears to run the flight. It’s the logo on the side of the plane you’re on. 

ticketing airline – the airline whose logo is on the ticket and the credit card bill