It's pretty much AT specific, though I hear it has bled over to the other "big" trails like the PCT and CDT.

However, for "shorter" long trails, like the John Muir Trail, Long Trail, or Colorado Trail, there is more ambiguity. Most would agree that if you don't stay in the general corridor, it's more of a section hike. Like if you hiked Denver to Salida on the CT, got off for a few weeks, than came back and hiked Salida to Durango, it would likely be considered more of a section hike even if it all happened over a mere 2-3 months.

For the BIG guys, there's a bit of leeway based on the fact that sometimes, the rest of the world intrudes on you. I got off the AT at Harper's Ferry for two weeks for a friend's wedding. Then I got back on and finished the remaining 2160 miles. Definitely considered a thru-hike.