ISAPI 
Internet Server Application Programming Interface 
An application programming interface that allows programmers to develop Web-based applications. These applications run more quickly than common gateway interface (CGI) programs because they use Microsoft Windows' Dynamic Link Libraries (DLLs), which run as part of the Hypertext Transport Protocol (HTTP) application's process. The necessary DLLs are loaded into the computer when HTTP is started and remain there as long as they are needed.