Atomic notes are notes that each hold a single idea, or knowledge building block, written so that the note can be understood, linked and reused on its own.
The idea is named in the 📝Zettelkasten tradition as the principle of atomicity. The Zettelkasten Method site, zettelkasten.de, traces the term to a September 2013 post on the site, which put it this way: "put things which belong together in a Zettel, but try to separate concerns from one another." The site's current guide calls atomicity "a processing direction in note-taking, aiming for one knowledge building block per note," and adds that "It is not a rigid law, but a guiding compass."
The method that principle grew from goes back to 📝Niklas Luhmann, whose card index Bielefeld University describes as about 90,000 notes written between 1951 and 1996.
Andy Matuschak carries the idea into his 📝Evergreen Notes, in a note titled "Evergreen notes should be atomic." He asks that each note be about one thing and capture that thing as completely as possible, and he treats the right size as a tradeoff with no clear rule: notes that are too broad blur their links, while notes that are too fragmented weaken the network of links as a whole. He compares the practice to separation of concerns in software, and quotes the zettelkasten.de formulation of atomicity as a source.