Almost anyone can open a dictionary, or browse to a website, to find a definition.
I mean, young children generally can't, given that they generally lack the motor skills and inclination. And someone without access to a computer would have a pretty tricky time browsing the web. But given these fairly basic premises, my point is that it's a pretty straightforward thing to do a bit of research about what other people mean by pie.
I've been doing just that, and have formulated my own conclusions. Because I can.
So, let's see what we have.
Firstly, let's think about what we mean by a pie, in terms as general as possible. For me, it's like a covered bowl, a shell that encloses and contains a secret, tasty filling. What forms that container is, however, the nub of the matter.
Obviously, there are cottage pies and shepherd pies (and the many variations on the theme), where the container is made of stoneware, and the lid is formed of mashed potato. As delicious as they are - and they often are - they're not what I crave when I fancy pie. If you promise me pie, and deliver me mashed potato, I'm afraid I'm going to be disappointed. I'll still love you, but my face will fall a bit. It's not you, it's me. That's just the way I am.
So we come to the pastry pie, which may be one-crust, where the lid alone is pastry, and a dish or a bowl forms the remainder of the shell (I have previously referred to this style of pie as the opposite of quiche). Alternatively, there is the tart (sometimes known as a quiche), where the pastry forms the container, but comes without a lid (or crust).
Again, as delicious as these pies-with-bits-missing may be, I do not consider them True Pies. They may aspire to pieness (they may indeed have had pieness thrust upon them), but they do not achieve it. Again, if I'm expecting pie, and I get one of these faux pies - Pie Lites, if you will - then there's going to be a certain amount of disappointment on my behalf. If you're going to serve a tart (or, daringly, a quiche), then by all means call it by its proper name. But let us not pretend that it is a pie.
For me, a pie must be two-crust, meaning that the pastry forms both the basin and the lid, completely enclosing the filling in pastry goodness (alas, too often, pastry badness is served). I can understand the reasons for the other variations and indeed enjoy them from time to time. But they lack that essential Pieness that I demand.
Pies, as I have observed before, are presents wrapped in pastry. When it's as simple as that, why change a thing?