In the Tomb of Annihilation adventure, I've encountered a specific instance where the term "Artifact" is used meaningfully (on p. 134):
A magical field around the statue attracts metal objects of any kind (not just ferrous metal). Any metal object that comes into direct contact with the shield disintegrates, showering the floor with powdered rust. Artifacts are immune to this corrosion.
Compare this to a different trap found in the module (on p. 138):
One can jam the propeller by fixing an immovable rod in place between two of the blades. Other magic items wedged between the blades are knocked aside as the propeller turns. The propeller destroys all nonmagical objects lodged between its spinning blades.
Comparing the two traps, it seems clear to me that the former trap destroys all but artifacts (magical items included). The latter only destroys mundane items (magical items immune).
Currently, my party is in possession of a Holy Avenger, which I would consider an artifact (however, that term is nowhere to be found in its description.) My party is also in possession of some magical, generic weapons that I wouldn't consider artifacts since they can be crafted. In my mind, the Holy Avenger wouldn't be disintegrated in the first trap due to its artifact status, but the latter would.
Colloquially, I've understood the term 'Artifact' to indicate either an antiquated, rare item or a magical item. However, after comparing the traps, I am lead to believe that there is perhaps a more specific designation for the word 'Artifact'.
Is there a definition for the term 'Artifact' in D&D? Is there a difference between an 'Artifact' and a magical item?