ELisp

ELisp, or Emacs Lisp, is the programming language used to configure and extend functionality in Emacs. ELisp is built off of Common Lisp and shares much of the syntax. ELisp was made specifically for the purpose of extending Emacs. Much of the Emacs source code is written in ELisp. Some of the low level functionality is built in C (like the interpreter for ELisp), but most of the built in libraries and functions are written in ELisp.