A note-taking system where each note was limited to the space of a physical 3x5 index card. Atomic Content Should Fit On A Single Page.
These Atomic Notes then had written links to other related notes, creating a Graph Database of interlinked ideas.
While the original Zettelkasten was physical, most Knowledge Management Tools these days are fully digital.
As a note tool for thought, it’s a bit like cultivating a Digital Garden more than it is writing a book.
Origin
Modeled after the human brain, (or rather, a model of the human brain), in which densely interconnected neurons become more than the sum of their parts, and Highly interlinked ideas naturally get repeated more often.
Invented by German sociologist Niklas Luhmann, who wrote over 70 published books. That, to me, is evidence of the effectiveness of this system.