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?
Best Answer
Artifacts are Magical Items; but only a tiny few of Magical Items are Artifacts
There's a few places in the rules where Artifacts are called out as being materially different than any other Magical Item, but nowhere as substantially as in the section in Chapter 7 of the Dungeon Master's Guide labelled "Artifacts":
Artifacts might have special properties
Below this section is a list of tables of possible special properties that Artifacts might have, ranging from benefits while attuned like increases to Ability Scores or gained Proficiencies, to possible detriments like attracting unwanted attention or an inability to safely handle jewelry.
Artifacts require a special process to destroy
Artifacts must be managed carefully by the DM
So aside from being exceptionally powerful, the presence of an Artifact implies some quite profound connection to the broader narrative of the campaign or setting. So regardless of what the Artifact is or what benefits/detriments it confers, it's important for any DM issuing an Artifact to make sure they understand the consequences it ought to have on their campaign.
Holy Avenger's are powerful magic items, but they are not Artifacts
Generally speaking, unless the DM is homebrewing an item and declares as such that an item constitutes an Artifact, items will have as part of their statblock a specific declaration that they are, in fact, an Artifact. This typically replaces the Rarity, which would normally otherwise range from "Not a Magic Item→Common→Uncommon→Rare→Very Rare→Legendary".
The Holy Avenger isn't even an item itself in its own right: it's a template that can be applied to a wide variety of weapon types, like a Holy Avenger Longsword or a Holy Avenger Greatsword. So even by the particular rarity of Legendary items, Holy Avenger is not especially unique, nor do items it has been applied to count as an Artifact.