Where does "mountain flying" begin?

At what point does uneven terrain become "mountain flying"? Do
rolling hills represent a problem? Is there a threshold of ruggedness
or altitude or some other distinctive characteristic of terrain (other
than variable elevation) that serves as a warning that mountain flying
skills will be necessary?

If you don't have mountain flying skills, and your departure and
destination are on opposite sides of a mountain range that is too long
to practically go around, what's the plan?

--
Transpose mxsmanic and gmail to reach me by e-mail.