Common Gateway Interface (CGI)
CGI stands for Common Gateway Interface. It is a protocol, whose specification enables web server to execute programs. CGI is platform independent, and can be used with any programming language.
PHP may be build as a CGI, but also as a webserver module or a CGI server.
CGI was widely used in the early days of the web, more modern web development approaches, such as server-side scripting have largely replaced CGI in contemporary web development due to performance and security considerations.