Mistborn trilogy spoiler warnings!
I don’t want to give too much away, but in Mistborn: Secret History, Khriss tells Kelsier, “Sixteen people, together, killed Adonalsium, ripping it apart and dividing its essence between them, becoming the first who Ascended.”
The shards therefore are the pieces of Adonalsuim, his attributes, his power, that people can hold and use.
Barring the Almighty, did we see a Shardholder (like Sazed) in TWOK?
I think “Shardholder” would get confusing alongside “Shardbearer.” Basically, in the Cosmere’s terms, when someone holds a Shard of Adonalsium, I call that person a Shard of Adonalsium. They are imbued with the power of that Shard, but they also become the Shard. Fans can use whatever terminology they wish, but this is how I term it.
You did at least see the direct effects of two of the Shards of Adonalsium, but I won’t say whether or not you actually saw a Shard of Adonalsium.
What is a Sliver?
Someone who has temporarily held a large part of the raw form of a Shard.
Would the Elantrians and the Lerasium-mistings be considered Slivers? Or is it just that the Lord Ruler and Vin are Slivers (via the Well of Ascension)? Or do you need even more power than that to be considered a Sliver?
Elantrians are not slivers. The Lord Ruler was indeed a Sliver. So was Vin. For the rest, I would say probably not.
What defines an actual Sliver of Adonalsium is not as clear-cut as you might think. It’s a term that in-universe people who study this have applied to various existences and states. Every single person on the world of Scadrial has a bit of Leras in them—a bit of the power of Preservation. Every single person has a bit of Ati in them. There’s a certain threshold where these scholars would call you a Sliver of Adonalsium. But I would say that any regular Misting is probably not a Sliver. A full Lerasium Mistborn is getting closer, but people who have held one of the powers are what would probably be termed a Sliver by the definitions.
If you hold all the power, that makes you a Shard. The Lord Ruler held a little bit of it and then let it go. From then on they referred to that change in him—the residue, what was left—as a Sliver. When he held it he became the Shard for a short time, and Vin was a Shard for a short time. After Vin gave up the power, what Kelsier is at the end of the trilogy—that’s a Sliver of Adonalsium.