Foreign Exchange Option

(commonly shortened to just FX option or currency option) is a derivative financial instrument where the owner has the right but not the obligation to exchange money denominated in one currency into another currency at a pre-agreed exchange rate on a specified date.

"The global market for exchange-traded currency options was notionally valued by the Bank for International Settlements at $158,300 billion in 2005.
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The FX options market is the deepest, largest and most liquid market for options of any kind in the world. Most of the FX option volume is traded OTC and is lightly regulated, but a fraction is traded on exchanges like the International Securities Exchange, Philadelphia Stock Exchange, or the Chicago Mercantile Exchange for options on futures contracts.