I've always thought the term 'Slack' country to be a funny one. I thought it meant backcountry in the proximity of a ski hill accessed with the assistance of a lift. The 'slack' part kind of conjures up images of laziness, but the sweat filling my eyes while hauling my body up Silverking Ridge seemed anything but lazy.
So I looked into it and it turns out there's a lot more specific terminology out there that describes the various combinations of lift access backcountry. Here's what I've found online, tetonat.com was especially helpful:
Slackcountry: terrain outside ski boundary accessed without bootpack/skin
Sidecountry: terrain outside ski boundary accessed with bootpack/skin
Frontcountry: anything you bootpack/skin and ski back to the road.
Backcountry: anything access solely by your own power.
Whatever you want to call it, some of the most fun I've had around Whitewater has been exploring the various types of 'country' the area has to offer. One short lift ride up and the world is your oyster. Here's a photo from a recent slackcountry mission on Scob's Knob…or was that sidecountry? Whatever you want to call it, it's always awesome.
It's worth mentioning, whatever kind of 'country' you're into, make sure you know the current avi conditions, have all the backcountry equipment necessary and don't go alone!