It’s the small individual idea in the Zettelkasten system. Niklas Luhmann started this and he wrote each idea on an index card organized inside the Zettelkasten. These were thematically unlimited and extendable to infinity in any direction. Especially now with Obsidian and the Second Brain, even more.
One idea per slip card. Luhman linked them if they are on the same topic or branch if it’s new ideas. As he read books or whatever, he would create new notes, update or add comments to existing cards or create new branches/links along the way. Each new note had a unique id.