The G5 (Group of Five) refers to the five largest Western economies: the United States, United Kingdom, France, Germany, and Japan. The group was historically influential in coordinating exchange rate policies, most notably with the Plaza Accord in 1985.
Example
“In 1985, the G5 nations signed the Plaza Accord, agreeing to devalue the U.S. dollar against the Japanese yen and German Deutsche Mark, causing major forex market moves.”