GNU Emacs Lisp Reference Manual

10.9: Buffer-Local Variables

Global and local variable bindings are found in most programming languages in one form or another. Emacs also supports another, unusual kind of variable binding: buffer-local bindings, which apply only to one buffer. Emacs Lisp is meant for programming editing commands, and having different values for a variable in different buffers is an important customization method. (A few variables have bindings that are local to a given X terminal; see Multiple Displays.)