What is a debt default? And the debt ceiling?

To default on a debt is to fail to make a scheduled payment on a loan. A sovereign debt default is when a country defaults on money it owes - as happened to Argentina in the early 1990s, for example.

What is the 'debt ceiling'?

The above video describes the political dimension of the debt ceiling. The US debt now stands at $21 trillion - see debt clock here. 

Some economists argue that this debt level is unsustainable in the long term. The key question Democrats and Republicans battle over is what to do about this.