In the Tomb of Annihilation advemnture, 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?

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":

An artifact is a unique magic item of tremendous power, with its own origin and history. An artifact might have been created by gods or mortals of awesome power. It could have been created in the midst of a crisis that threatened a kingdom, a world, or the entire multiverse, and carry the weight of that pivotal moment in history.

—Artifacts, Dungeon Master's Guide, pg. 219

Artifacts might have special properties

Each artifact has its own magical properties, as other magic items do, and the properties are often exceptionally powerful. An artifact might have other properties that are either beneficial or detrimental. You can choose such properties from the tables in this section or determine them randomly. You can also invent new beneficial and detrimental properties. These properties typically change each time an artifact appears in the world.

—Artifacts, Dungeon Master's Guide, pg. 219

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

An artifact must be destroyed in some special way. Otherwise, it is impervious to damage. Each artifact has a weakness by which its creation can be undone. Learning this weakness might require extensive research or the successful completion of a quest. The DM decides how a particular artifact can be destroyed. Some suggestions are provided here:

• The artifact must be melted down in the volcano, forge, or crucible in which it was created.


• The artifact must be dropped into the River Styx.


• [...]

—Artifacts, Dungeon Master's Guide, pg. 221

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.

Artifacts are a kind of magic item

In the DMG p. 219, they are described as:

An artifact is a unique magic item of tremendous power, with its own origin and history.

In terms of how to actually know if an item is an artifact, the item's description will say so where a magic item would list their rarity. For example, the Book of Exalted Deeds has the following definition (emphasis mine):

Wondrous item, artifact (requires attunement by a creature of good alignment)

In fact, over on DnD Beyond, artifact is listed as a magic item rarity. Note this filter listing all artifacts on DnD Beyond

It's about the uniqueness of the item and they follow the description in the DMG

Artifacts, are described in Chapter 7 of the DMG and provides a list of some of them. There may be other Artifacts provided via other modules, but they should be labeled as such.

If the item is made by a DM, then it'll be up to the DM to determine if it's an Artifact or not.

But note that(emphasis mine):

An artifact is a unique magic item of tremendous power, with its own origin and history. An artifact might have been created by gods or mortals of awesome power...

Characters don’t typically find artifacts in the normal course of adventuring. In fact, artifacts only appear when you want them to, for they are as much plot devices as magic items. Tracking down and recovering an artifact is often the main goal of an adventure. Characters must chase down rumors, undergo significant trials, and venture into dangerous, half-forgotten places to find the artifact they seek. Alternatively, a major villain might already have the artifact. Obtaining and destroying the artifact could be the only way to ensure that its power can’t be used for evil.

Items not in the DMG can be compared against the above to see if they qualify.

Holy Avenger is NOT an Artifact

The Holy Avenger is listed in the general list of Magic Items in the DMG. More specifically, it is not listed in the Artifacts. That combination of facts leads to the conclusion that the Legendary item is not an Artifact.