Currency Pair
Every price in the foreign exchange market is expressed as a pair because money has no absolute value — it only has value relative to something else. When you see EUR/USD, you are looking at a single number that tells you how many US dollars one euro costs. The two currencies are inseparable; changing one automatically changes the other.
The pair is always written in a fixed order by market convention. The first currency listed is the base currency and the second is the quote currency. Suppose EUR/USD is quoted at 1.0800 — that means one euro buys 1.08 US dollars. If the pair rises to 1.0900, the euro has strengthened against the dollar, not the other way around.
A common confusion is thinking the slash means division in an arithmetic sense. It does not — it is just notation identifying which currency is being priced (left) and which is doing the measuring (right). You can explore how these quotes are read in practice in the how to read FX quotes guide, and track live rates across the currencies data page.