coin of the realm

1. The coins used in monetary exchanges in a particular country. Drachmas are no longer the coin of the realm in Greece. You'll need euros if you want to buy anything here. A: "What are these, Grandpa?" B: "Rubles—the coin of the realm when I was a boy in the Soviet Union." Do I need to exchange money before I go to Canada? What's the coin of the realm there?

2. By extension, something that is especially valuable in a particular field or area. You better get this unique idea of yours patented—intellectual property is the coin of the realm for inventors. Financial support from the board is the coin of the realm if you want to keep your research project going. You got the CEO's endorsement? Wow, that's the coin of the realm if you want to get anything done around here.