A published module or set of modules that provide several different adventures, even a whole campaign, and often spans a large amount of character advancement. Term often associated with Paizo's Pathfinder.
Adventure paths are a collection of dungeons, towns, cities and an overarching campaign that brings all these elements together to create one part of a living breathing roleplaying world. They typically span a massive chunk if not all of the character advancement available to the player, depending on the Adventure Path in question.
The term is often associated with Paizo Publishing, as they publish many Adventure Paths.