When currencies are traded or exchanged in the foreign currency markets, they are traded in pairs, but in a scenario where the US dollar is paired with another currency, we have or refer to this currency pairing where the US dollar is not the base currency as a reciprocal Currency.
A reciprocal currency is a case where another currency is the base currency and US dollars us the second currency. Example is AUD/USD. Here is dollars is the quoted or second currency.