Abbreviation of 外国為替 (がいこくかわせ). Used in financial contexts, often in compounds like 外為市場 (foreign exchange market).
See also: 外国為替
The yen is strengthening in the foreign exchange market.
Abbreviation of 外国為替 (がいこくかわせ), combining the first characters of 外国 (foreign) and 為替 (exchange).