What is a good definition for the term "Airline"?
The term is used extensively, and it's meaning is well understood at the one end of the spectrum: air carriers flying passengers on large aircraft on scheduled routes. However, the other end of the spectrum seems to be far less defined or understood. This question is a good case in point: Why don't we have helicopter airlines?
I am primarily interested in exploring the term from an operational standpoint, not a entitative standpoint, but both are valid.
I am familiar with the FAA's 14 CFR 1.1 definitions of Air Carrier and Commercial Operator, but I am not aware of any FAA provided definition of Airline. I am asking this definition from my background of knowledge of FAA regulations. However, I would be interested in an answer that would be valid worldwide.
Criteria to consider include the following, all of which apply to Air Carrier operations (some of these criteria overlap):
A Single pilot company, vs a multiple pilot company
Passenger carriage vs freight
Scheduled vs on-demand
§135, vs §125, vs §121, etc.
Aircraft category— fixed wing vs rotorcraft
Aircraft size— number of passenger seats, pounds of cargo payload
Common carriage vs non-common carriage or contract carriage