What's the definition of an artifact in regards to a Sphere of Annihilation?

Question

A sphere of annihilation erases all matter except for artifacts. What exactly would the prerequisites of an artifact be for it to be an exception to the Sphere's effect?

From the sphere's description:

The sphere obliterates all matter it passes through and all matter that passes through it. Artifacts are the exception. Unless an artifact is susceptible to damage from a sphere of annihilation, it passes through the sphere unscathed. Anything else that touches the sphere but isn't wholly engulfed and obliterated by it takes 4d10 force damage.

Answer 1

Artifact is a rarity type in D&D 5e. As you can see in this filtered list from D&D Beyond, artifacts are the next level of rarity up from Legendary.

There isn't a specific guide to what makes an artifact an artifact, but by scanning the list, it becomes clear that these are one-of-a-kind items, frequently associated with iconic and powerful characters from various settings and adventures. Meanwhile, the list of legendary items contains various items that are incredibly powerful, yes, but most of them are items that there could feasibly be more than one of in existence.

Artifacts are incredibly powerful when equipped and attuned by players, which means that it'd be a tragedy if someone accidentally destroyed one with the Sphere, but more importantly, many of them are big evil macguffins. Allowing the Sphere to destroy them would cut many a quest quickly short!

For example, the Sword of Kas is an artifact that can only be destroyed by someone who possesses both the Eye and Hand of Vecna; this is a two-part artifact that imposes serious alignment issues on the attuned party. Allowing a single legendary item to destroy the sword would be anticlimactic to say the least.

Answer 2

An Artifact is a special category of magic item, as described in the Dungeon Master's Guide on page 219.

Specifically, they are immensely powerful, completely under the DM's control, and half magic item, half plot device.

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

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.

So the only real prerequisite is "does the DM consider this object an artifact?"

The book lists a few samples, but really just encourages the DM to come up with their own objects of awesome power. As such, the only real way to find out is asking them, although your character is probably almost immediately aware that something is an artifact on account of these things being mythical/legendary objects.

Answer 3

An artifact is an unique magic item of divine (or equivalent) power

Mechanically speaking, "artifact" is special category of magic item. In addition to being the absolute highest level of rarity, even rarer than "legendary", there are also special rules that apply to all artifacts, most of which are detailed in Chapter 7 of the DMG. The most relevant to this question are the rules for destroying an artifact:

An artifact must be destroyed in some special way. Otherwise, it is impervious to damage.

The special way to destroy a given artifact is detailed in its item description (though this information might not be known to the player characters). So, unless "throw it into a sphere of annihilation" just happens to be the special way to destroy a particular artifact, that artifact cannot be destroyed, or even damaged, by a sphere of annihilation nor any other mortal magic, no matter how powerful. This kind of exception is common for artifacts. For example, Antimagic Sphere suppresses all magical effects and spellcasting within its area, except for "those created by an artifact or a deity".

Lore-wise, most (all?) artifacts are items created directly by a deity or something of equivalent power, which explains why they are impervious to things that would suppress, damage, or destroy most magic spells and items. They are, in the most literal sense, "god-tier magic items".