mod_lisp is an extension module for the Apache HTTP Server that enables Apache to interface with web applications written in Lisp. This makes it possible to dynamically generate web pages and provide web services with Lisp programs.
Unlike other "mod_" languages, such as mod_perl and mod_python, mod_lisp does not embed a Lisp interpreter in Apache. Its purpose is to handle requests by converting them into simple text format and sending them over a socket to a Lisp system, and then sending the Lisp system's response to a client. In this respect, it resembles FastCGI.