What is the difference between custard pie and custard tart?
Pies originally were specifically to denote enclosed items (the crust sealed the item that was to be eaten).  In many cases, the crust wasn't actually eaten -- it was a nasty charred thing that was discarded.

In time, pie crusts improved to the point at which you'd eat the whole thing ... but the star was the filling, not the crust.

Tarts, on the other hand, are closer to a shortbread cookie with some sort of a topping.  The crust is typically a critical part of the item, rather than just being a container for something else.  (which in part is why people mention tarts being more shallow; there's a higher proportion of crust to filling).

So, my best attempt at determining if I should call something a pie or a tart.  Note that these are in order.  (as I'm trying to make sure that 'pizza' is a pie, not a tat)


fully enclosed by crust : pie
full bottom crust, with a crumb or lattice top : pie
no crust at all : not a tart, but not a pie, either.
has no sugar or fat in the crust : pie
chewy, flaky or crumb crust : pie
not structurally sound enough to be picked up and eaten one-handed : pie
was cooked in a tart pan : tart
baked without a pan : galette
over 2" / 5cm tall (not counting whipped cream or berries) : pie
less than 6" / 15cm across : tart
shortbread crust : tart


... I have no idea what I'd categorize thing that didn't match something already.