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An *IDL* is an *interface definition language*.  The most well-known is CORBA.

An IDL compiler takes a specification and generates stub code that hides the
transport details, and by this implements a [[RPC]] system.

In the case of [[Mach's MIG|microkernel/mach/mig]], this hides the marshalling
and unmarshalling of parameters according to [[microkernel/Mach]]'s semantics,
and invoking the respective [[microkernel/mach/port]] operations.