An interesting question and thread and a question for which I believe there is no precise formal answer simply a range of views based on personal opinion and common usage. I haven't reviewed the main dictionaries but I suspect they will all have slightly different definitions and these will also change with time.
I suppose an definitive answer has to contain some form of qualifier to allow the definition to vary as accepted use will depend on context. To the owner of a Spitfire the definition might well (although not exclusively) be relatively narrow. To the owner of an L4 the definition might be broader.
My personal definition is anything which was designed with the military in mind and I can afford!. Currently therefore I include the Chipmunk as this had a military purpose (albeit training). In the future if I am able to progress to sexier machines I might recalibrate my decision to exclude training aircraft and only include those with a front line role.
So for now I am with the EAA who allow Chipmunks to use the warbird arrivals procedure at Sun'n'Fun.