Base currency
The base currency is the first currency in a pair. It is the one you buy or sell, and the quote shows its price in the second currency.
In GBP/USD, the British pound is the base currency. The quote, say 1.2700, tells you how many dollars one pound is worth. The base currency always counts as one unit in this calculation.
When you buy 1 lot of GBP/USD, you buy £100,000 and sell the equivalent in dollars. Lot size is therefore always expressed in the base currency, which matters when margin is calculated.
The order of currencies follows market convention, not relative strength: the euro comes before the dollar, and the dollar before the yen or the Swiss franc. You won't find USD/EUR at your broker.
See also