Through-hiking, or thru-hiking, means hiking from one end of a long-distance trail to the other. If you start at the trailhead, visit whatever your destination may be and then retrace your steps back to that first trailhead, you haven't through-hiked -- you've done an out-and-back.
You'll most often see terms like through-hiking and section-hiking (hiking a long trail a piece at a time) applied to long-distance hikes that measure in the thousands of miles long, like the Pacific Crest Trail and the Appalachian Trail. But both terms also apply to shorter trails that take more than a day to complete.
It's a question of logistics...
Those who do commit to one of the longer through-hikes may be on the trail for months at a time. They usually resupply themselves with the help of prearranged shipments to mail drops along the trail. (Exactly how they access those resupply packages determines whether it's a supported or unsupported hike.)
The really hardcore can do a "yo-yo": Hiking all the way from one end of the trail to the far end, then hiking all the way back to where you started from.
Through-hikers tend to become ultralight backpackers -- or at least very, very efficient in their systems -- out of sheer necessity. When you must carry everything you need on your back between resupply stops that may be more than a hundred miles apart, you learn how to make do and when to make do without.
...and for some, it's a lifestyle
Exactly how much you can deviate from the established trail during a through-hike, without losing your "through-hiker" status, is a deep, philosophical question -- I'll let those of you with strong feelings about it decide that one.
The answer probably has as much to do with why you're on the trail in the first place as anything else. Suffice it to say that some long trails have designated bypasses to help you get around the harder sections if you so choose, and of course you can create your own at need. Also, some of the longer through-hikes actually incorporate walking along the roadside as part of the trail (if no better option is available).
Examples of famous through-hikes in the United States include:
- The Pacific Crest Trail: This 2,650-mile trail runs from Mexico to Canada by means of California, Oregon, and Washington. For an interesting (and very personal) account of through-hiking this trail, read "Wild: Lost and Found on the Pacific Crest Trail," by Cheryl Strayed.
- The Appalachian Trail: Also known as the AT or simply "The Trail," this 2,180-mile trail winds from Georgia to Maine.
- The Continental Divide Trail: This 3,100-mile trail traces the Rocky mountains from Mexico to Canada. Through-hiking these first three trails is known as the "Triple Crown."
- The Mountains-to-Sea Trail This 925-mile trail in does exactly what it sounds like, taking you from the mountains down to the sea, all within the bounds of North Carolina.
- Superior Hiking Trail: Abundant no-fee campgrounds and trailheads every 5 to 10 miles make this 286-mile trail in Minnesota a great place to start your through-hiking career.
- John Muir Trail: This 215-mile trail starts in Yosemite and ends at 14,496-foot Mount Whitney. Most of it overlays the Pacific Crest Trail.
- Tahoe Rim Trail: This 165-mile trail traverses the peaks around Lake Tahoe. It's actually a loop that passes through California and Nevada; 49 miles of it coincide with the Pacific Crest Trail.
- Outside Online has basic information on a few lesser-known through hikes.
Alternate Spellings: thru-hiking
Common Misspellings: thru-hiking, thru hiking, through hiking
Examples: "Dude, she just through-hiked the Appalachian Trail. I'm impressed!"