Zettelkasten

From Wikipedia:

The zettelkasten (German: "slip box", plural zettelkästen) is a system of note-taking and personal knowledge management used in research and study.

This post on the r/Zettelkasten subreddit has a great introduction to the system, but here's a short snippet:

The term comes from Niklaus Luhmann, a German autodidact and famously prolific academic sociologist. Similar techniques were developed independently by Nabokov and Prisig, among others.

Luhmann used a physical box of cards -- imagine an analog library card catalog. Most modern practitioners use digital slip-boxes. The core of the technique lies in a few simple principles:

• Write liberally about what you think and read. Write in detail so you can quickly re-internalize ideas after you forget them.
• Wherever you can, separate notes into small and distinct ideas.
• When you notice connections between ideas, encode them by writing references from one note to another.
• When you want to think or write, read your notes and follow the references between them.