Artifact is a catchall term for any number of mysterious objects with effects comparable to magic, but don't appear to be powered by aer. No one is quite sure where these Artifacts came from, who or what made them, or even how many of them there are. There is speculation that they're affiliated with various Divines or Fiends, but the Church has never confirmed nor denied those theories.
An example of an Artifact is a pair of earrings with teleportation capabilities, allowing the owner of one to teleport to the other, and vice versa.
All Artifacts are collectively referred to as being part of "Pandora's Box."