What is a tax deficiency?

Tax deficiency is defined in section 6211(a) of the IRS Code. Tax deficiency occurs when the correct amount of income tax owed is more than the amount shown on the taxpayer's return. This means that the taxpayer still owes tax to the IRS and might be assessed interest and/or penalties. If the taxpayer disagrees with the IRS, the taxpayer should provide the IRS with documented proof to support this.