What is the US debt ceiling and what happens if it isn’t raised?

The US government has hit its borrowing limit, known as the debt ceiling - here's what that means

The US government has hit its borrowing limit, known as the debt ceiling, marking the beginning of what looks to be a vicious fight over the government's budget and one that threatens to worsen an already precarious economic outlook.

The US treasury secretary, Janet Yellen, has said extraordinary measures" are now being put in place to prevent the United States from defaulting on its obligations - essentially moving some money around so the government does not default just yet. Those measures will last a few months, but if the limit is ultimately not raised, the federal government will run out of funds.