At what point does an aircraft become an airliner? For me, the term 'airliner' conjures up an image of a passenger jet that is capable of hosting a large number of passengers¹ but I'm finding it difficult to source a reputable definition or etymology despite the term has been sourced int the early 20th century.

Since the term has been in use since 1908, jet as in jet airliner may be inferred by the modern user but it obviously wasn't part of the original definition.

The word itself would seem to indicate that airliner refers to an aircraft used by an airline (a company that provides air travel for the public), but would that extend to bush pilots with small prop-driven Cessnas or passenger helicopters?


  airliner - A large passenger aircraft. A large plane for carrying passengers:
  a commercial/commuter airliner &nbsp;&nbsp;source: Cambridge English Dictionary
  
  airliner - An airplane operated by an airline &nbsp;&nbsp;source: Merriam-Webster Dictionary


&nbsp;&nbsp;Is this an 'airliner'? How many passenger seats need to be added before it becomes one?&nbsp;&nbsp;(image courtesy of The Bush Pilot Company)

The term seems so vague that even the airliner tag on this site only uses 'large' to define size or capacity.

Some reference work definitions include cargo aircraft but the predominant emphasis seems to be on passenger aircraft with no upper or lower limit criteria.

I suppose that this term could be relative to the observer or situation but if the term 'airliner' was used in FAA regulations then I'm sure there must be an appendix somewhere that provides a definition. My research has come up empty.

Apologies if this is a dumb question and the term is intentionally meant be vague with only 'large aircraft' or 'many passenger' as criteria. Given proper references as support, that could be an answer.



¹ This previous question got me wondering exactly what was the criteria before any aircraft could be called an 'airliner'.
I think there's a misunderstanding here that airliner ("an airplane operated by an airline") is directly derived from airline. While it is a reasonable definition, "airliner" predates airline. (Mirriam Webster: airliner 1908, airline 1910). 

An airliner is a portmanteau of air and liner, a liner being any passenger ship plying between ports along regular "lines" (as distinguished from transient ships using those ports). By the early 20th century a liner was generally understood to mean any large, passenger-carrying vessel operating a route between major ports (think Titanic etc.). You'll notice how airliners travel between airports - nautical words and phrases were adopted in the early days of flight.

So an airliner should be


passenger carrying
large 
travels between airports, rather than having one fixed point of operation


It's a definition that still fits, and covers any commercial aircraft over a certain size. While there's not a specific point at which an aircraft becomes large enough to be called an airliner, I think it does impart some sense of largeness or capacity that an 'ordinary' plane does not possess.

(You might even say that a better definition of airline is "an air transportation system that operates airliners"!)