Base Currency
The first currency quoted in a currency pair on forex. It is sometimes referred to as the primary currency.
Investopedia Commentary
For example, if you were looking at the CAD/USD currency pair, the Canadian dollar would be the base currency and the US dollar would be the quote currency.
Related Links
A Primer On The Forex Market
Floating And Fixed Exchange Rates
Forces Behind Exchange Rates
Using Currency Correlations To Your Advantage
See also: Currency, Exchange Rate, Forex, Quote Currency