I would define an IFR flight as: "A flight for which an IFR flight plan or IFR flight itinerary is filed."
I say that because I could follow all the same procedures and rules paco mentioned while VFR.
So this probably shifts your question to "what are the conditions under which I am
legally required to file an IFR flight plan?"
That would be:
* in weather below VFR minima
* in class A airspace, or
* under any conditions where your company ops manuals require it
Part of the confusion of "IFR" is that the same term is used to describe the weather, the rules, the flight plan, the aircraft cababilities and the pilot's qualifications.