G-20 Leaders Vote for CCPs: What are they?

In September 2009, as the financial crisis was starting to slowly recede, the leaders of the twenty largest economies of the world (the G-20) met in Pittsburgh. At the end of their summit, they issued a communiqué of nearly 9000 words. Somewhere in the middle of the statement, the following sentence appeared:

All standardized OTC [over-the-counter] derivative contracts should be traded on exchanges or electronic trading platforms, where appropriate, and cleared through central counterparties by end-2012 at the latest.