Gaming is an innovative industry by nature, with every game the way we make games improves along with the quality of the games themselves; but through this innovation we also push boundaries and push into new genres, and when these new games are hard to define, we often lump them in as 'Action-Adventure game.'
Wikipedia defines action-adventure games as, you guessed it, a game with the qualities of an action game as well as an adventure. It defines an action game as a game that requires quick thinking, skill, and reflexes; while adventure titles feature puzzle and role-playing elements that need more careful planning as well as a story centric gameplay. Now that seems fair as a definition until you realize that every game on the market features opportunities for planning and story as well as fast paced combat encounters. The action-adventure genre literally covers every game out there (with the exception of a few purist adventure indie games.)
The main difference between adventure games and easier to define RPGs is adventure games don't give the player as much choice. Adventure games might feature open worlds, branching paths, etc., but every playthrough is more or less the same, where as an RPG features varying storylines and encounters based on your choices.