The Memex is a hypothetical device described by Vannevar Bush in "As We May Think" (The Atlantic, July 1945): a mechanized private file and library in which an individual stores books, records and communications for fast, flexible consultation.
Bush wrote the essay as Director of the U.S. Office of Scientific Research and Development, urging scientists to turn after the war to making the growing store of knowledge more accessible. He argued that conventional indexing is artificial, while the human mind works by association. He coined the name "memex" himself, by his account at random.
As described, the memex is a desk with slanting translucent screens, a keyboard and sets of buttons and levers, with its contents stored on microfilm. Users buy content on microfilm or photograph their own notes into it through a transparent platen.
Its essential feature, in Bush's account, is associative indexing: any item can be made to select another immediately and automatically. With a single key the user joins two items, after which either one recalls the other. Many joined items form a named trail that can be reviewed, branched with side trails, and copied into a friend's memex. Bush foresaw encyclopedias arriving with associative trails already running through them, and a new profession of "trail blazers" who build useful trails through the common record.
The memex is an early design for what are now called 📝Tools for Thought (TfT), and its joins work in both directions, as 📝Bidirectional Links / Backlinks do in later software.