Mxsmanic wrote:

At what point does uneven terrain become "mountain flying"?

it is a bit arbitrary; in my neck of the wood, the local
FBOs and clubs tend to set a limit at airports which elevations
are above 2000'; at or above that they require a 'mountain
check out' -- and happily recognize check outs made in other
clubs/FBOs. In my case the check out consisted in spending
a really nice day landing and taking off to/from eight airports
in the Sierra Nevada with an instructor, and it was a lot of
fun :-)

but to answer your question: you could define mountain flying
as operating from any place where the effect of high density
altitude is significant; But this is only one point (albeit a
really important one); actually you don't even need to
go high to get the effect of high density altitude: flying
in the desert in summer can be just as challenging (if not
impossible at some time; part of the check out consists in
showing the the pilot understand the performance charts and
effects of density altitude in the go/no-go decision);

then, you have additional things to take into account due to
terrain, understanding things like waves, convection, where to
expect downdraft, more advanced stuff like canyon flying, etc.

and then you have the really wild altiports as it is called in
some places (I believe that in France, some of these airports
in the Alps require specific checkouts/endorsements -- something
I'd love to do one of these days);

--Sylvain