A coin pair, in the context of cryptocurrency trading, refers to the comparison of two cryptocurrencies' prices. It's like an exchange rate, showing how much of one coin you can trade for another. For example, the BTC/USD pair indicates the price of Bitcoin in US dollars. Popular pairs often involve Bitcoin (BTC) due to its dominance in the market. Other common pairs include ETH/BTC (Ethereum vs. Bitcoin) and USDT/BTC (Tether vs. Bitcoin). These pairs are crucial for traders as they help analyze price movements, volatility, and potential trading opportunities within the cryptocurrency market.