Federal Debt Limit
The debt ceiling is the overall limit on federal government borrowing, as authorized by Congress. It is similar to an individaul's credit card limit.
According to the Constitution, the Congress must approve all borrowings on behalf of the United States. Before the 20th centurY, Congress approved all bond issuances separately and explicitly. With the introduction of the debt ceiling, the treasury now had a line on which it could borrow as it needed to without having to go back to Congress for borrowings under the ceiling.
Barack Obama voted against raising the debt ceiling in 2006.