I was blissfully ignorant until I found this topic. I had always assumed that it suggested doing one of the "big three", but really had no idea.

Web definition

"Thru-hiking is the process of hiking a long-distance trail from end to end. The term is most commonly associated with the Appalachian Trail, but is also used for other lengthy trails and long distance hikes, including the Pacific Crest Trail and the Continental Divide Trail."

en.wikipedia.org/wiki/Thru-hike

Also included is hiking the Appalachian Trail end to end within one years time.

So I'm assuming that the white blaze and blue blaze terminology also comes from the Appalachian Trail?

And Brian, what is "slack pack"?

It seems reasonable to me, given the effort logistical and otherwise that needs be done to complete one or more of the "big three", to give it a special name such as "thru hike". It is a jump in the level of commitment needed compared to other backpacks. I have done the JMT a few times and have never considered it a thru-hike, although this is probably because I never gave the terminology of what I was doing much thought. Basically I was there to enjoy without analysis.