What is the definition of QE2?

Quantitative easing (QE) is a monetary policy used by some central banks to increase the supply of money by increasing the excess reserves of the banking system, generally through buying of the central government's own bonds to stabilize or raise their prices and thereby lower long-term interest rates.

The '2" in QE2 can simply be addressed as "the second round" or second in a series of action(s).

OR

HMS Queen Elizabeth 2 (QE2), cruise ship of the Cunard Line; launched 1967, retired 2008.