A key concept of the Zettelkasten method, permanent notes are the bricks out of which an edifice of knowledge is built. Unlike transient notes you might take while reading a book and then never come back to, permanenet notes aspire to continued relevance for as long as you remain interested in their topic, and should be revisited and developed as your understanding of their subject deepens. And unlike permanent publications (e.g. essays, papers), permanent notes are relatively short, focused on a single idea, and likely to change over time.
Andy Matuschak calls these “evergreen notes” and argues that they are the fundamental unit of knowledge work:
If you had to set one metric to use as a leading indicator for yourself as a knowledge worker, the best I know might be the number of Evergreen notes written per day.