What is the legal definition?
Sorry to be lazy.
Thanks,
IMHO

That is the fundamental problem.  Congress didn't define it so the ATF gets the opportunity.  In my meager understanding of law, they are supposed to provide a definition and enter it into the administrative records.  (Code of Federal Regulations or some other referenced document)  Unfortunately, they have not done so to my knowledge, which means we must all ask for technical reviews.

Enter the vagueness doctrine.  For laws to be valid, the average person needs to be able to understand them.  What's the chance of that if there's not even a law to look up?  Seems to me that several Federal firearms fall into this category.  e.g.  What's an "unsporting feature"?

Thanks, Southern_Raider.  

As far as the technical reviews go, is anyone aware of the criteria by which the device is defined?  Is there a threshold for suppression of flash?  Sounds difficult to define, but there must be something like this, right?

I realize that something advertised as a "Flash Suppressor" is a "feature," but I wonder how the manufacturers make the distinction.  If there really is no referencce, then it seems that it would be an advantage to the producers of the devices to call everything a muzzle brake.  This way, the market for the product could increase, right?  Wrong?

Who knows?