A Zettelkasten (German: 'slipbox', plural Zettelkästen) or card file consists of small items of information stored on Zetteln (German: 'slips'), 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.
In the 1980s, the card file began to be used as metaphor in the interface of some hypertextual personal knowledge base software applications such as NoteCards. In the 1990s, such software inspired the invention of wikis.