I always understood that a thru-hike (note spelling!) is a hike from one end of a really long trail (at least 100 miles) to the other. Of course living close to the PCT I tend to think of the Triple Crown trails, but there is a longer one (American Discovery Trail) and lots and lots of shorter ones, some under 100 miles. There are even more trails planned, but many of those are still mostly in the wishful thinking stage.

While a thru-hike is usually considered completing the trail in a single year, you can get full credit for completing a trail--even the Triple Crown trails--by section hiking. It just takes longer!

I've never heard anyone describe the Wonderland Trail around Mt. Rainier (almost 100 miles) as a thru-hike. Is that because it's a circle? Or is it a product of the PCT's being so close?

For me, with my 5-6 mile per day pace, there's no way I'm going to thru-hike anything! Plus I much prefer the enjoy-the-view, stop-and-smell-the-flowers approach to hiking. That doesn't mean I don't admire and respect thru-hikers, though! It's just a different life style.