Exactly what makes someone an adventurer is a little ineffable, at least inside the fantasy of the game. Outside of the game the definition is easy: adventurers are characters being controlled by a player.

But in-world what exactly defines an adventurer? Is it going on adventures? Maybe explicitly taking quests or helping the common man? But, to a goblin tribe, the gobos raiding the human settlement for food and safety would be pretty darn heroic. And taking on a whole village of people twice your size certainly sounds like an adventure.