In U.S. politics, a government shutdown is the process the Executive Branch must enter into when the Congress creates a "funding gap" by choosing not to or failing to pass legislation funding government operations and agencies, or, after the Congress passes a bill to fund the government and sends it to the President, the President vetoes that bill.
If interim or full-year appropriations are not enacted into law, the United States Constitution and the Antideficiency Act require that the federal government begin a "shutdown" of the affected activities. If the funding gap lasts long enough that shutdown plans must be enacted, the law requires the furlough of non-essential personnel and curtailment of agency activities and services. Programs that are funded by laws other than annual appropriations acts (like Social Security) also may be affected by a funding gap, if program execution relies on activities that receive annually appropriated funding. Although the term government shutdown usually refers to what occurs at the federal level, shutdowns have also occurred at the state/territorial and local levels of government.
Since 1976, when the current budget and appropriations process was enacted, there have been 18 gaps in budget funding, seven of which led to federal employees being furloughed. During the Reagan administration, there were three funding gaps leading to shutdowns lasting less than 48 hours or over weekends. A funding gap during the George H. W. Bush administration also caused a weekend shutdown.
During the Clinton administration, there were two full government shutdowns during 1995 and 1996 lasting 5 and 21 days respectively, the longest and most severe to that date. These shutdowns led to massive furloughs and significant disruption. The primary issue was the United States budget deficit.
During Barack Obama's presidency, the United States federal government shutdown of 2013 ran from October 1 to 16, 2013. The primary issue of dispute between the Republican-controlled House of Representatives and the Democratic Senate was the Republicans' desire to delay or defund the Patient Protection and Affordable Care Act (Obamacare), signed into law in 2010. A bill to end the shutdown and fund federal agencies through January 15, 2014, passed the Senate and the House and was signed into law on October 17, 2013. Standard & Poor's, the financial ratings agency, stated on October 16 that the shutdown had "to date has taken $24 billion out of the economy," and "shaved at least 0.6 percent off annualized fourth-quarter 2013 GDP growth." Wikipedia