Second brain
A second brain is a personal knowledge management system, typically a network of notes, sources, and links, that captures what you read, organises it for retrieval, and surfaces connections between ideas. The phrase was popularised by Tiago Forte and overlaps with Zettelkasten and digital-garden traditions.
A working second brain has four functions: capture (save what you encounter), organise (give it structure you can navigate), distill (reduce noise to signal), and express (turn notes into output). Most failures happen at distill, capture is easy, structure is fun, but compressing notes into reusable insight is the hard part.
Tools matter less than habits. The same Zettelkasten can run on Obsidian, Notion, Roam, or Atlas, the differentiator is whether the tool surfaces existing notes when you write new ones. Without that surfacing, the system collects but does not compound.
AI changes the equation: a second brain that can answer questions about its own contents, with citations, is qualitatively different from one that can only be searched by keyword.