The IRS released audit statistics of its activities in the government’s fiscal year ending September 30, 2010. It shows that the number of returns that were examined increased by 11%; 8% of all such returns were taxpayers with taxable income of $1 million or more. (Almost 389,000 taxpayers had taxable income of $1 million or more.) Overall, the IRS examined more than 1.58 million returns in its fiscal year 2010, compared with 1.43 million in the previous fiscal year.
For entities, the rate of audits was up for charities and other tax-exempt organizations, but down slightly for corporations and partnerships.
Source: http://services.taxanalysts.com/taxbase/eps_pdf2010.nsf/DocNoLookup/26745/$FILE/2010-26745-1.pdf
The excess of the tax assessed by the IRS over the amount reported on your return.