The first currency in a pair, the one being priced.
Example
In EUR/USD, the euro is the base currency.
Important
Buying the pair means buying the base currency and selling the quote currency.
Related terms
Currency Pair — Two currencies quoted against each other, showing how much of the second is needed to buy one unit of the first.
Quote Currency — The second currency in a pair, in which the price of the base currency is stated.
Lessons that explain it
Every currency trade is a bet on two countries at once — A forex position is always one currency against another. Retail traders usually hold it through leveraged broker products, while every international investor already holds currency risk without trading at all. (9 min)