As for "official" I mean published by the official owners/publishers of D&D (vice a second or third party).
Even that's controversial - Paizo did three Adventure Paths in Dungeon (Shackled City, Age of Worms, Savage Tide). By the terms of their license, these were "100% official" D&D (not d20) adventures.
I believe that since the term was coined, there have been five and a half paths published:
The first path, from "Sunless Citadel" to "Bastion of Broken Souls"
The three aforementioned Dungeon paths
The H1-3/P1-3/E1-3 path for 4e
The "Scales of War" path currently running in eDungeon.
Prior to that, there was the GDQ1-7 path, and also a significant number of trilogies (or more) of adventure modules. I'm not sure if any of those are really long enough to be full-blown paths, though.
Unofficially, we have "War of the Burning Sky" (actually, 2 of these), "Rise of the Runelords", "Curse of the Crimson Throne", "Second Darkness", "Legacy of Fire", "Council of Thieves" (in progress), plus at least one trilogy of mega-modules from Mongoose, and probably some others I'm not aware of.