How would you define Interoperable?

Looking at interoperability. Very crudely so that an end user gets no surprises. Unable to find a definition that is usable as a measure! Google has a few, typical abstract form. None much practical use though.

How would you define it so that it could be used to define a test? A classic would be a Word Perfect file import into Word. Room for lots of misinterpretations IIRC. That was a weak interoperable format when I last used it (long time back).

Someone suggest an XSD schema and I'll ignore them!

The only good example I've seen is a cross head screw and screwdriver. Slice it carefully and you get a good example of interop.

Not something I've worried about before and it has me scratching my head.