Answer:
Adjusted Gross Income
Source:
U.S. Tax Policy Center
Last paragraph states:
"We estimated the cost of Senator Obama's proposals assuming that the Social Security proposal would impose a 2 percent income tax surtax on adjusted gross incomes over $250,000 and a 2 percent payroll tax paid by employers on employees' earnings above that threshold and that all of the provisions-including the higher payroll tax-are fully effective immediately."