An abstraction could be a design requirement, expressed in some way.

That's definitely an abstraction.

IDLs are a form of abstraction, they're a requirement somewhat more formally described.

Remember UML? That was an attempt to go 1/2 layer above the code, that was an abstraction.

There were tons of tools like that.

APIs are an abstraction - maybe the best example. We write code to match exactly the behaviour defined by an APU - as long as it meets the requirement of that contract, then 'it's good'. And there could be many ways of doing that.

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