Русско-англо-русский юридический словарь

In the forex market, currency unit prices are quoted as currency pairs. The base currency – also called the transaction currency - is the first currency appearing in a currency pair quotation, followed by the second part of the quotation, called the quote currency or the counter currency. (Investopedia)

In terms of foreign exchange trading, currencies are quoted in terms of a currency pair. The first currency in the pair is the base currency. The base currency is the currency against which exchange rates are generally quoted in a given country. Examples: USD/JPY, the US Dollar is the base currency; EUR/USD, the EURO is the base currency. (Moneycontrol)