Short for “Interface Control Document”. This is a document (usually a Word file or PDF) that describes interfaces in detail. In this context, it describes software interfaces. It sets expectations about connecting, transmitting data, data types, formats, commands, and more. It is THE handbook that should be the source of truth. Unfortunately, keeping the ICD and the latest versions of software current with each other is exceedingly difficult and the ICD can sometimes lag reality.

Modern teams have been moving to database driven solutions so there’s a central, single “source of truth” that can be used both for documentation and during the compiling process.