I define it the same way that the legal profession does. Put it to a jury. Let them decide whether the tax-return is a valid representation of the company's tax affairs, or whether they are fabricated financial structures designed to avoid the paying of taxes that parliament and the treasury intended and expect companies to pay.
If we decide whether someone goes to jail by presenting the jury with evidence and them deciding if a law was broken, why can't we do the same with CFOs and blatantly tax-evading tax returns?
Interesting world you live in... where if someone does something you don't like... they should be thrown before a jury to decide if it was actually illegal or not... never mind the costs involved (on both sides of such a system.
Unrelated, you have routinely offended my sensibilities on this site, I demand that you be hauled before a jury to answer for your 'crimes'.
First amendment you say? Isn't it for the jury to decide if you are actually violating some law which may be invented or discovered later.