A semi-mythical organisATion of wizardly LISP and Scheme hackers. The name refers to a mAThemATical formalism invented by Alonzo Church, with which LISP is intimATely connected. There is no enrollment list and the criteria for induction are unclear, but one well-known LISPer has been known to give out buttons and, in general, the *members* know who they are. [Jargon File]