Central bank

A central bank is a national bank that issues a country's currency, regulates the national money supply and regulates a country's interest rates. Central banks implement a government's chosen monetary policy and are often also tasked with regulating the country's private banks. Usual goals for monetary policy are high employment rate, price stability, and economic growth among other things.

Of course as they belong to the State and have a monopoly on money printing among other things they're not liked by a number of libertarians. You know, those who complain that because of their existence the epoch of the Industrial Revolution was not one of deregulated economies, ignoring why Communism and Socialism appeared in that epoch.

Use by cranks
Cranks on both sides of the political spectrum seem to be obsessed with central banks, especially the Federal Reserve, with a multitude of conspiracy theories TRUTHS!

In many countries, including the US, central banks are supposed to be operationally independent - something which certain Tea Party influenced US senators don't seem to grasp.