But that is exactly why we need to define the different transit categories and differentiate between them:
Do we though. By defining categories, we reject the possibility that it's a continuum undefinable by categories. For example, one might claim that the REM is light-metro west of Bois-Franc, but is behaving more like a heavy-metro east of Bois-Franc.
Line 4 is heavy-metro - but it's got the same length trains and a similar capacity for the underground section of the Eglinton line between Mount Dennis and Laird. It may even be more frequent. And it's similar to the Montreal Blue line in terms of train lengths and capacity when the Blue line was exclusively used 6-car trains for many years last century.
Therefore, I would rather define a "metro" as an "electrical railway, which operates entirely grade-separated and on dedicated tracks not shared with other urban or interurban rail modes".
Which, like all definitions, is going to require exceptions. Such as the pieces of the London Overground, which are surely metro, but use diesel. Or the Chicago El line with the level crossing. And isn't there track sharing on parts of Seoul Line 1 towards Suwon and beyond? And then, what if VIA does get it's way, and shared the Mount Royal tunnel with the REM (which I think is unlikely)?
This makes the Métro de Montréal, Toronto's Subway, the Scarborough RT and the future REM network a Metro, whereas exo, GO Transit, UPX, the Toronto Streetcar, and any LRT scheme in planing or operation in the GTHA I'm aware of are urban or suburban rail networks.
I'm very suspicious of any scheme that labels the low capacity Scarborough RT (The Scarborough Skytrain!) as Metro, but labels the much higher capacity Line 5 west of Laird as something else!
The terms are interesting - though often academic. Wikipedia has been a battleground of terminology - particularly between North America and the rest of the world. The principle there is common usage prevails - the challenge is whose common usage.
But to follow Wikipedia's example - in local articles, you use local terminology and common usage. And this is not only a Toronto-based forum ... it's an urban Toronto (as opposed to suburban!) forum. So Toronto terminology should predominate, though with preference for local English terminology used elsewhere when discussing there systems. So Toronto and New York have subways. Montreal, Washington and Paris have metros. London has tubes and undergrounds (and trams and overgrounds!). Meanwhile Montreal has chosen to brand REM as light rail (though I can see it could also have been branded metro or RER), as has Ottawa. The jury is still out, particularly with the increasingly bizarre recent schism between TTC and Metrolinx, but it looks like they'll brand Line 5 as subway, like Line 3.
In Toronto, Metro is a grocery store, and Subway is also a sandwich shop!
If we ignore labels, and focus on capacity, frequency, and travel times, the discussion becomes simpler.