Unfortunately congress has no power to define the term. Only the Supreme Court can make it settled law. It is specifically derived from the constitution. (Not even case law can settle it.)
So tell me what the definition is, then?
One of the proper uses of the “necessary and proper’ clause (which has been used in many improper ways) is to define terms not specifically defined, to determine just what these terms do and do not mean.
That’s a Congressional power. The Founders never intended to give that to the courts.