What defines a "thru-hike"?

A better question might be: who defines a thru-hike?

There is no organization that rules on these things, so there is going to be a touch of anarchy about the whole enterprise. Which, considering the activity and who does it, seems to me only apt.

If the "who" is me, then I would allow that anyone who walks from the starting point to the end point, by whatever route, should be able to consider themselves a thru-hiker. Most large-scale human endeavors have an amount of uncertainty and "slop" connected to how they are finally accomplished. A few miles dropped out for sane reasons shouldn't negate the whole of the accomplishment.

As Napolean's right hand general (Ney) said, the first casualty of any battle is the battle plan. Getting all "purist" about who is a thru-hiker is most often about raising yourself in your own eyes by denigrating someone else. Why one path (white blazes) is real and the other (blue blazes) is not real has nothing to do with hiking and everything to do with mind games.

btw, I'm a westerner who's hiked maybe 15% of the PCT overall. I've never set foot on the AT.