Demon
In Ars Magica, a demon is any creature from the Infernal Realm. Statistics for three example demons are given in the ArM5 rule book; many more are described in Infernal.
In real history, it's pretty clear that the medieval Church believed demons were real. In Mythic Europe, demons are demonstrably real through such powers as Sense Holiness and Unholiness. Hermetic magic is unreliable at detecting demons due to the Hermetic Limit of the Infernal.
The extent to which demons are prevalent and influence Mythic Europe is a choice for individual troupes to make for their own sagas. A lot of Third Edition supplements focused heavily on demons and infernal corruption; some might say they over-did it.
References
- Infernal Creatures, ArM5 page 196
- Limit of the Infernal, ArM5 page 80
- The Infernal Legions, Infernal pages 28-49