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 s

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.