Booker C. Bense
If you're using your own effort to get up the hills and you're
not skiing on a prepared track, that's backcountry skiing. Covers
everything from a short jaunt in your back yard to skiing down
Everest.

Martin Thornquist
I more or less agree,

I think I'd agree completely with Booker here.

but what do you call xc skiing on groomed tracks in the
backcountry?

Track skiing. Modern groomed tracks are a far cry from natural
conditions. Modern tiny XC racing gear excels on these, but,
depending on the snow conditions, going off the track with
such gear is hopeless.

Perhaps a bit like the difference between road cycling and
mountain biking. Except bigger.

Speaking of cycling, a prepared XC-skiing track (skating style) is
great for cycling. In summer too, btw.