Common Gateway Interface. It's a specification for how web servers can process HTTP(S) requests dynamically.

EDIT: If you are wondering what modern equivalents are, AWS Lambda[1] and Azure Functions[2] are Liskov substitutable[3] as they pertain to CGI programs. Both of these products provide benefits beyond what a "classic CGI system" could offer, of course.

0 - https://en.wikipedia.org/wiki/Common_Gateway_Interface

1 - https://docs.aws.amazon.com/lambda/latest/dg/welcome.html

2 - https://docs.microsoft.com/en-us/azure/azure-functions/funct...

3 - https://en.wikipedia.org/wiki/Liskov_substitution_principle