Term used to describe an arrangement where one airline sells seats (the marketing carrier) on a flight operated by another airline (the operating carrier). Both airlines display their respective flight numbers. This is particularly common within airline alliances, such as Star Alliance. eg. Qantas operates QF1 but codeshares this flight with Britis [..]

A codeshare is a flight segment that is sold under the flight number of one airline, while being operated either partly or entirely by another airline. Both companies maintain their independent profiles in the market.

means a flight where the Airline Designator Code in the flight number is not the Airline Designator Code of the Carrier operating the aircraft

[verb] Nefarious activities that got the likes of Robert Hanssen, Aldridge Ames and Edward Snowden in big trouble.  

Airlines that are partners, but not members of the same airline alliance. For example, American Airlines and Ethiad have a codeshare agreement and their miles are redeemable for flights on each other, even though they’re not in the same alliance.