And how is it different from a longbow? Or can a longbow be a flatbow...? Or a flatbow a longbow? ...how much wood can a woodchuck chuck....
Our British friends call our longbows (Hill, etc.) American flatbows to differentiate them from the English longbow, which has a D shaped cross section. I think in common usage, a flatbow has wider limbs and possibly shorter limbs than a longbow.
I'm sure there will be an expert or two along shortly to explain it properly.
long bows English have a D cross section, flat bows are flat on the bellie side like a American Indian bow. they can be from 72" down to 35" in length.
Scattershot pretty much summed it up, here in the UK we already have a "longbow" of our own, the American bows are therefore called American Flatbows.
However it gets complicated by the fact that pretty much any of the decent American bows are not allowed in the "Flatbow" Class for 3D shooting as they tend to have reflex/deflex. There are a fair number of English bowyers producing flatbows which have the continuous curve in the limbs which meet the requirements (Border being one of them).