"Interface Definition Language" refers to any of several Programming Language-independent syntaxes (MS-IDL, OMG-IDL, etc.) for describing object interfaces in component or Remote Procedure Call systems.
The interfaces have attributes and operation signatures. Each interface specified in the IDL specifies the operation signatures through which requests are made.
An IDL is only a definition language; mappings to Programming Languages are provided for invocation and manipulation. Some IDLs support interface inheritance.
CORBA (Common Object Request Broker Architecture) OMG-IDL:
Microsoft COM (Component Object Model) MS-IDL:
Remote Procedure Calls (RPCs) in Open Software Foundation's Distributed Computing Environment (DCE):
SUPPORT IN DCE RPC IDL -- www.opengroup.org 
(introduction) www.linuxjournal.com
RPC IDL:
ISO/IEC 14750:1999 Information technology -- Open Distributed Processing -- Interface Definition Language -- www.iso.org
ISO/IEC JTC1 SC32 -- www.iso.ch
CapIDL (an Object Capability Model IDL used by Eros Os, Coyot Os and Ee Language):
www.capidl.org (that site has succumbed to domain squatter use coyotos.org instead)
See also Design In Idl
See original on c2.com