$BTC

In cryptocurrency trading, a BTC pair refers to a trading arrangement where Bitcoin (BTC) is paired with another cryptocurrency or fiat currency for trading. Essentially, it's a way to trade one asset against Bitcoin, rather than using traditional currencies like USD. For example, ETH/BTC means you're trading Ethereum (ETH) using Bitcoin as the base currency. 

Here's a more detailed explanation:

Trading Against Bitcoin:

Instead of trading a cryptocurrency directly against a fiat currency like USD, you can trade it against Bitcoin. 

Base and Quote Currency:

In a trading pair, one asset is the "base" currency (in this case, BTC) and the other is the "quote" currency (e.g., ETH in ETH/BTC). 

Examples:

ETH/BTC: This means you're trading Ethereum against Bitcoin. 

BTC/USD: This is a Bitcoin-US Dollar pair, where the price is quoted as how many US dollars it takes to buy one Bitcoin.