Merriam-Webster's Word of the Day for May 19, 2019 is:

coin of the realm • \KOYN-uv-thuh-RELM\  • noun phrase

1 : the legal money of a country

2 : something valued or used as if it were money in a particular sphere

Examples:

The coin of the realm changes from one country to the next, so travelers may turn to digital transactions through services like PayPal.

"The 'game' is to see who ultimately will rule from the Iron Throne. This addictive game often plays out like a suspenseful succession of high-stakes chess moves…. There are kings and queens, knights and pawns maneuvering for position, forming strategic alliances on these fictional continents where danger, duplicity, deception and deceit are the coin of the realm."
— Mark Dawidziak, The Plain Dealer (Cleveland, Ohio), 14 Apr. 2019

Did you know?

Coin of the realm gained currency in the English language during the 18th century as a term for the legal money of a country. Coin is ultimately from Latin cuneus, meaning "wedge," and entered English, via Anglo-French, in the 14th century with the meaning "cornerstone" or "quoin." By the latter part of that century, the word was being exchanged as a name for a device or impress stamped on flat pieces of metal used as money and, by extension, for the money itself. Realm entered English in the 13th century with the meaning "kingdom." Its spelling is an alteration of Old French reiame, which is based on the Latin word for "rule" or "government," regimen. In time, realm was generalized as the name for any sphere or domain, and coin of the realm came to signify something having value or influence in a particular sphere.