There are many different types of terminology in Konosuba covering various subjects.
Eris (): Eris is the name given to currency in Konosuba, named after the Goddess of the same name. They are minted with copper, silver, gold, and mithril. Each Mithril coin is worth 1,000,000 Eris, and according to Aqua 1 Eris is roughly the same value as 1 JPY.
Keele's Dungeon (キールのダンジョン): Keele's Dungeon is a single-floor underground dungeon created by the arch wizard Keele long time ago. It's a beginner dungeon that can be reached in a few hours after departing from Axel. It is a fairly popular training spot for beginner adventurers, as it is relatively easy and located just a short distance away. However, since it's pitch-black inside, you will need to use night-vision or some lighting skills to explore the dungeon.