Budget sequestration, or simply "sequestration," is an automatic reduction in planned spending by the federal government set to begin March 1, 2013. These cuts total merely $1.2 trillion over 10 years, which would barely affect the overall national debt of $16 trillion. Moreover, the cuts are primarily to planned growth in future spending, and may not reduce current spending levels much at all. These cuts are to be divided 50% for domestic discretionary spending (not affecting entitlement programs like Social Security), and 50% for defence spending.
The term is formally defined in the Budget Enforcement Act (BEA) as the cancellation of budgetary resources provided by discretionary appropriations or direct spending laws. New budget authority, unobligated balances, direct spending authority, and obligation limitations are “sequestrable” resources; that is, they may be subject to reduction or cancellation under a presidential sequester order. 
The Executive impoundment of Congressionally mandated funds, i.e., a president's refusal to spend money appropriated by Congress, had a long history. But after President Richard Nixon refused to spend money appropriated by Congress for Johnson-era Great Society programs, Democratic President Jimmy Carter signed legislation passed by the Democratic controlled Congress making it illegal for a president not to spend money appropriated by Congress. By 1985 it became apparent some form of budgetary restraint was necessary. Sequestration was designed as a deficit-reduction forcing mechanism, so that unless the President and Congress cooperate each year to reduce the deficit by prescribed amounts, a “sequestration” order would reduce funds down to a mandated figure.
Example of sequester
|“||Obama is a very strategic thinker. He knew precisely what it would take to get elected and didn't blow it. He used community organizing methods... And I agree with this strategy. Anything else will court sure defeat. Move on the stuff you can to a small but significant extent, gain support and confidence. Leave the military alone because they're way too powerful. For now, until enough momentum is raised. By the second or third year of this recession, when stimulus is needed at the bottom, people may begin to discuss cutting the military budget... ||”|
On December 5, 2012 it was reported that the Pentagon on instructions from the White House Office of Management and Budget began planning to use sequestration to cut Congressionally mandated military and defence spending.