Zettelkasten

https://en.wikipedia.org/wiki/Zettelkasten

A Zettelkasten (German: ‘slipbox’, plural Zettelkästen) or card file consists of small items of information stored on Zetteln (German: ‘slips’; singular: Zettel), 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.