Strictly, any program which handles its input and output data according to the CGI standard. In practice, CGI programs are used to handle forms and database queries on web pages, and to produce non-static web page content.
A program that uses the common gateway interface (CGI) to perform tasks not usually done by the server, such as form processing. CGI programs can be written in any language supported by the operating system on which the server is run. The language can be a scripting language or a programming language.