Acronym for the Common Gateway Interface
The CGI standard specifies the conditions for programs run by a web server in respons to certain queries in order to return a webpage. It specifies things like EnvironmentVariables such as PATH_INFO, QUERY_STRING, REMOTE_USER, and many more which you can read to find out information about the request. It also specifies where a CGI programs STDIN etc should be connected and a few other details of operation.
4 pages link to CGI:
lib/main.php:944: Notice: PageInfo: Cannot find action page