What defines a "thru-hike"?

Is it walking past all of the blazes, never skipping or bypassing any?

Is it walking continuously from one end to the other (I.E., not flip-flopping)?

Is it following the general "gist" of the trail, but changing routes slightly (such as following another, nearby trail for a distance before returning to the main trail)? By this I mean following the general trail corridor but perhaps not the footbed itself so religiously?

I've read that purists deem that anyone "blue-blazing" the AT hasn't actually hiked the AT; one must hike by every white blaze within a calendar year. I've read that folks on the PCT or CDT are a bit more relaxed about that. Is it true?

What are your thoughts, and why?
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"Stand in the ways and see, and ask for the old paths, where the good way is, and walk in it; then you will find rest for your souls."