Design Flattening

I don't think there is a universal definition. At one time many, many years ago, synthesis didn't do too much across hierarchies. By flattening, you essentially got rid of those hierarchical boundaries and could get better results. The downside is that you often lost the hierarchical naming, which made the final results harder to understand when debugging. As far as I know, all synthesis tools work nicely across boundaries and there really is no need to try and flatten a design.