e.g. 'flaming', 'fast', 'sarcastic'
Traits are codified properties of the Object that make it unique and interesting. They could be physical descriptors, intangible properties, reputations, etc.
They can have subtext and deeper meaning for the player, for example the Trait 'Legendary' will necessarily be supported by the events of their story. Since this is a single-player experience, it's not as important for this deeper meaning to be recorded explicitly.
Each kind of magical item supported by the game has a list of 20 or so sensible Traits for that archetype, but players are always free to make up their own.
For example, the Weapon has the following traits:
Flaming, Frost, Corrosive, Silent, Warding, Dimensional, Vampiric, Fragrant, Spectral, Gelatinous, Warped, Blessed, Vindictive, Fickle, Loyal, Curious, Knowing, Devious, Romantic
Twenty is a good baseline for each Object's list of suggested Traits, but more or less might be needed. The more you provide, the more prescriptive you will seem about what Traits are appropriate for that Object. However, too few might lead a player to struggle to come up with original Traits or end up with a more generic selection.
Grouping Traits as you list them can also be helpful for the player, for example by starting with physical descriptors before moving on to properties, personalities or reputations.
The order of Traits can also subtly suggest the shape of the Object’s story. For example, you could offer a spread of broadly positive Traits followed by more negative ones, which might suggest decline as a potential theme in the Object's story.
Finally, consider how much guidance a player might need to generate Traits. Artefact is very light & freeform in this regard, but you might want to introduce more constraints—choosing from a fixed list, mechanising their meaning (e.g. positive/negative), etc.