zettelkasten
A Zettelkasten or card file consists of small items of information stored on paper slips or cards that may be linked to each other through subject headings or other metadata such as numbers and tags. It has often been used as a system of note-taking and personal knowledge management for research, study, and writing. Wikipedia
The building a second brain and modern personal knowledge management movements are inspired and informed by the Zettelkasten practice pioneered by german sociologist @Niklas Luhmann. He credited this flat nonhierarchical database of notecards as the key to his prolific writing and publishing, enabling to more easily recognize the connections across domains of knowledge.