The opening paragraph of the Home Page of the Department of Justice's Tax Division:
The Tax Division’s mission is to enforce the nation’s tax laws fully, fairly, and consistently, through both criminal and civil litigation, in order to promote voluntary compliance [emphasis mine] with the tax laws, maintain public confidence in the integrity of the tax system, and promote the sound development of the law.
WARNING: I am not advising anyone reading this blog not to pay their taxes. All I've been saying, and what Irwin Schiff and others over the years have said, is that the government refers to the individual's filing of a tax return as voluntary compliance.
[Thanks to Douglas Roudybush]