Base currency
The first currency in a pair; the quoted price tells you how much of the second currency one unit of it costs.
In a currency pair written as XXX/YYY, the base currency is XXX. The price always reads “one unit of the base equals this many units of the quote”. In GBP/USD 1.2700 the base is the pound: £1 costs $1.27.
The base matters for reading direction. When the pair rises, the base is getting stronger against the quote; when it falls, the base is getting weaker. A Higher trade on USD/JPY is therefore a view that the dollar will gain against the yen — or, equivalently, that the yen will lose against the dollar.
Which currency comes first is a matter of market convention, and the conventions are historical rather than logical. The euro is the base in every pair it appears in; the pound comes before the dollar; the dollar comes before the yen, the Swiss franc and the Canadian dollar. That is why you see USD/JPY but EUR/USD.
A frequent slip is reading a rising USD/CHF chart as the franc getting stronger. It is the opposite: the franc is weakening. Read this entry together with quote currency and currency pair.
USD/CAD moves from 1.3600 to 1.3650: the base currency, the US dollar, has strengthened — one dollar now buys 1.3650 Canadian dollars instead of 1.3600.