What term describes the operation of an airline between two or more points?

The term that describes the operation of an airline between two or more points is "flight." A flight encompasses the journey taken by an aircraft as it departs from a specific origin and arrives at one or more destinations. This operational aspect is fundamental to how airlines function, as each flight represents a segment of scheduled service that connects various points on a route.

In contrast, a carrier refers to the airline itself, emphasizing the organization that operates the flights rather than the act of transporting passengers or cargo between locations. An itinerary, on the other hand, is a detailed plan of travel, usually including scheduled flights, dates, and times, but does not define the operation itself. Aircraft substitution involves replacing one aircraft with another for a specific flight due to various factors, but it does not describe the broader operational concept of connecting points through flights. Thus, the focus on the journey aspect makes "flight" the most appropriate term for this operation.