You asked for legalese, and as the IFR flight plan/itinerary is a necessary and sufficient condition for an IFR flight, that defines IFR flight.
The other uses of "IFR" are informal. People will ask if a pilot "has his IFR". That means they want to know if he has an instrument rating. To file IFR, your instrument rating must not have expired and you must have flown 6 hours actual or simulated IMC and done 6 approaches to minima in the last six months. Your instrument rating must also match the airplane you are flying in, with respect to number and placement of engines and nationality.
For an airplane to "be IFR" it needs to have the equipment listed in CARs 605.18.
If "the weather is IFR" that means it is instrument meteorological conditions (IMC). The conditions defining IMC are laid out in 602.114 and 602.115. If the weather is below those minima, you are technically in IMC. For example, if you are at flying at 2000' agl, with a cloud deck 400' above you, you are technically in IMC. If you're not on an IFR flight plan/itinerary you're technically VFR in IMC. I say "technically" because when most people say "in IMC" they mean you're looking out the window and seeing nothing but the grey (or black, at night) insides of clouds.
You could also have a pilot and aircraft that were IFR capable, but not be able to file IFR because your company's operating certificate only approves VFR operations.
Your VFR "definition" isn't defining, however. The term you are trying for when you said "visual flight conditions" is VMC -- visual meteorological conditions. But have you heard the expression "continued VFR"? It's short for "continued VFR flight into IMC," and you see it on accident reports. If you were VFR and had the weather close in on you, so you decided to fly an IFR altitude, on an airway, you would still be a VFR flight. VFR in IMC is illegal and dangerous, but shows that a definition of VFR based on VMC is not accurate.
That really is the only legal difference between an IFR and VFR flight: the flight plan. Remember that the R in IFR stands for rules, and there are a LOT of them.
On a VMC day, you could file IFR between two adjacent airports. Your IFR clearance could ask you to maintain runway heading to some altitude, and then contact the approach controller for the destination airport. That approach controller could then clear you for a visual approach. You would have done nothing different on that flight from someone flying the same hop VFR, but you'd be IFR because of your flight plan.
Near the end of your posting you said these definitions were for non-flyers. That makes it easy. Forget all this and just tell them "IFR flights are allowed to go into clouds but VFR flights have to stay certain distances away from clouds."
And for your PS: yes, CVFR is only relevant in class B airspace. If you're obeying a controller's instructions in class C or D, you are not in CVFR flight.
Now, are you more or less confused?
