A second brain for AI is a personal knowledge store that you and your AI tools can both read, keeping context outside any single chat.
The phrase 'second brain' comes from personal knowledge management, where it means a trusted external system for notes, ideas and references. Applied to AI, it means the same store is also readable by assistants and agents. Instead of re-explaining a project to ChatGPT, then to Claude, then to Copilot, you keep one set of notes and files, and each tool reads from it. The person owns the store. The tools are visitors.
This differs from built-in chat memory in an important way. ChatGPT and Claude memory live inside one vendor's account and cannot be read by other tools. A second brain sits outside all of them. It should be inspectable (you can open the files), portable (you can move or back up the folder) and durable (it does not disappear if a vendor changes plans). Basic Memory and Central Brain both take this file-first approach; Notion AI treats the Notion workspace as the brain.