Krugman: What does "printing money" mean?

..these days — with credit cards, electronic money, repo, and more all serving the purpose of medium of exchange — it’s not clear that any single number deserves to be called “the” money supply...

But if you’re determined to view economic affairs through a sort of paleo-monetarist lens, focused on the evils of “printing money”, you’re going to have a hard time in the modern world, where the definition of money is increasingly vague. [source]