No, voila, a boundary! A room is bounded in 3 dimensions. A boundary is bounded in 2, and is much more difficult using Hermetic magic - increased magnitudes, requires a ritual, cannot be used in an item.
If a troupe allows an open courtyard to be a "room" using a house rule, more power to them, but formally it would require either a boundary or convincing a Columba to do the work.
It seems to me that most troupes do not consider the existence of a roof as necessary for a Room. There does seem to be an agreement that the existence of a roof can turn a non-Room into a Room - for example a roof supported by short columns at the corners would make a Room even when the four columns on their own would not.
Excuse me, but could you please provide a source for your claim that a 'Room' (in the specific context of Ars Magica and Targets) requires being bounded in 3 dimensions?
Just about nothing in Ars Magica really takes #D thinking into account. Which is why we've had so many incarnation of that debate about Circles being spheres or cylinders.
I'll just quote:
All that is required, is a reasonably clearly defined boundary (as opposed to Boundary sigh) in 2 dimensions. In some ways, I'd have prefered if your definition was correct, since it would void silly things "it's not a T: Boundary, it's just a very large Room!", but what you're writing just isn't in the definition.
Now, I've intentionally been avoiding this conversation, because I'm a little unclear on the exact definition of a 'glade' in English. As far as I can tell, it is defined as "an open space in a wood or forest."? It is distinct from being a 'clearing' in that a clearing is artificial?
However any open area in a forest is likely to have fairly well-defined borders, since that's where you find bushes and undergrowth, plants that couldn't get any sunlight under the canopies of other trees. This has been described as a 'Wall of Green'. When you don't see these, it's usually because people have intentionally (and some would say "artificially" been clearing such growths away.
I would happily call an actual glade (assuming I've understood the word correctly) a 'Room, for this purpose. with relevant modifiers for size etc.
That's probably because they referred back to ArM5, p. 113, as they should.
A courtyard would often count, a valley would not.
Often, not always. You would still need some feature in the surrounding enclosure to set height, even if irregular. A room, by definition, "must be enclosed and have definite boundaries." No enclosure, no room. A ruined or partial roof would count--the enclosure doesn't need to be airtight and without openings, else you couldn't have windows or doors in your room. But Stonehenge wouldn't count, as it doesn't have roof and has never pretended otherwise. Unless you are arguing that a stretch of pavement with a few benches around the edge would be a room, and a town square with some shrubbery at the edges would be a structure. Respectfully, that's daft.
Courtyards don't have roofs, so a roof can't be a requirement for something being a Room.
So, four walls and no roof can be a Room. This probably requires that the walls are reasonably high compared to the area they enclose. (A low fence alone doesn't make a Room)
Stonehenge likely wouldn't count - the "walls" have much empty space, and are too low compared to the interior area. If, however, one could add a roof to Stonehenge, it probably would count - a roof would make it far more "room-like".
Or at the very least, they don't have them often, so "a courtyard would often count" seems to imply that some Rooms have no roof.
For what it's worth, note that to determine the size of a Room, only the floor area counts, so this in some way detracts from the notion that a roof is mystically important - in my view.
Some courtyards qualify, some do not. I think this is an important thing to note. As courtyards aways seem to have boundaries and needn't have tops, that seems to mean which ones would or would not count would based upon
it must be enclosed
Based on this, some courtyards must not be enclosed enough, while others are. I've never seen a courtyard even vaguely enclosed as little as Stonehenge or ezzelino's photo of cypresses. If those are even less enclosed than the least enclosed courtyard, then I don't see how they could qualify if less-enclosed courtyards don't qualify. Now Troy's picture of the tree-tunnel I could see qualifying; of course, that wouldn't fit the definition of a "glade" as far as I'm aware, but Room doesn't care about that definition.
Not by my usage of English (which can be idiosyncratic at times). I think a glade is natural, a clearing may or may not be. So every glade is a clearing, but not the other way round.
The other difference in my head is that a glade has more positive associations. I think of a glade as having seasonally appropriate wild flowers. While a clearing could easily have a rusty fridge.
I used "Glade" to mean a not too large clear space surrounded by forest.
And was open to the possibility there might be differences between a natural clearing and a man-made one.
Why do you want a definition? Ars Magica is story telling. It is play with language. And we do not normally understand words in terms of definition. Understanding evolves from use and experience. If it feels like a room, it is a room as far as the story is concerned. Overthinking it spoils the story whatever the conclusion.
You could play Ars Magica as a tactical game, building on formal language, mathematics, and engineering, but it is not what it was made for.
My apologies for complicating the question then 
While we are clearly in the "it-depends-on-the-individual-troupe" zone, I think it's worth noting that there are (at least) two aspects to "being enclosed".
The first is how "porous" the enclosure is. A solid wall is less porous than a fence, which is less porous than a row of columns, which is less porous than the row of pickets of a fence (without the fence). As I mentioned, in our troupe anything as porous as Stonehenge or a row of columns just fits the bill, because people actually tend to see those porous barriers as walls of a sort, and hence architects since antiquity have widely made use of them as such.
The second is how "complete" the enclosure is. I've seen plenty of courtyards that are enclosed only on three sides, and quite a few that are enclosed only on two (picture two square buildings, with a square space between them, all squares of the same size). Again, in our troupe, as a rough rule of thumb about three quarters of the perimeter must be "walled" (even if with a "porous" wall, see above) for a courtyard to qualify as a Room.
You have never seen the fenced backyard of an archetypical English terraced house, have you?
Trust me, if that's not a room, a room does require a roof.