A reference currency is the currency used as the yardstick for another value. A fiat-backed stablecoin that aims to be worth one US dollar has the dollar as its reference currency; under MiCA, an e-money token must reference a single official currency. A country running a currency peg or currency board fixes its exchange rate to a reference currency, often called the anchor currency, such as the dollar or the euro.
The term is also used in finance more broadly. Investment funds report performance in a reference currency, and investors holding them in another currency face foreign exchange risk. Card networks and payment providers may convert a transaction through a reference currency when no direct rate exists between the two currencies involved. In each case the reference currency defines what "stable" or "unchanged" means for the value being measured, and it should not be confused with the quote currency of an exchange rate.