Short for Microsoft Interface Definition Language, MIDL defines interfaces between client and server programs. Microsoft includes the MIDL compiler with the Platform SDK to enable developers to create the interface definition language (IDL) files and application configuration files (ACF) required for remote procedure call interfaces and COM/DCOM interfaces. MIDL also supports the generation of type libraries for OLE Automation. Create client and server programs for heterogeneous network environments that include such operating systems as Unix and Apple. [Source: MSDN Library - MIDL]
See also IDL.