All dungeons in the game are called Instances. This is a term used to describe a location that exists outside of the normal game environment, enclosing the player(s) in a location that seals them off from outside interference by other players.
Some dungeons have several types of instance, and other locations are simply known as "an Instance". For example, there are parts of the tutorial that are completed in the normal game world, and other parts that are completed as a single-player instance.
For each dungeon in the realms of Incartum, there is an option to progress as an Instance-Matched Party, or as a single player. IM dungeons are more difficult to complete but yeild the highest rewards, where single player instance dungeons are far easier and good for completing quests.