It's any real time RPG that uses a straightforward [ie, hack and slash, shooter, combo] combat system. Top-down Loot-driven Hack&Slash action-RPGs [like you just listed] are a common association, but games like Dragon's Dogma would also count. (More)
Basically, nearly any game with the tags "Action" and "RPG" would typically qualify.

[P.S. An example of a non ARPG hack & slash would be God of War (More).]

As a bonus, here's the wiki description on it:

Action role-playing games (abbreviated action RPG, action/RPG, or ARPG) form a loosely defined subgenre of role-playing video games that incorporate elements of action or action-adventure games, emphasizing real-time action where the player has direct control over characters, instead of turn-based or menu-based combat.

Oddly, afaik nowhere in the original info released on the game was that part of the title, and it's not part of the title on the official site, or on GOG.

Seems to have just been a weird Steam publishing decision.
Perhaps to make it stand out more to ARPG fans, due to how easy it is to get lost amidst the flood of games on Steam?