Which term relates to anything involving two or more airlines?

The term that relates to anything involving two or more airlines is "Interline." Interlining occurs when airlines collaborate to provide a seamless travel experience for passengers connecting from one airline to another, often using a single ticket. This arrangement allows for coordinated schedules, baggage handling through multiple airlines, and simplified check-in processes. It is essential for creating a unified transportation network, enabling passengers to travel across different airlines without facing significant disruptions or challenges.

The other terms, while relevant to various contexts within aviation, do not directly pertain to the cooperation between airlines. Involuntary re-routing refers to the changes made to a passenger's travel itinerary due to operational needs, while a left behind passenger pertains to someone who misses their flight. An inadmissible passenger relates to those who are not permitted to enter a country due to legal or immigration issues. These contexts are more specific and do not encompass the collaborative nature of interline agreements between airlines.