The $BTC coin pair refers to any cryptocurrency trading pair that involves Bitcoin (BTC) as one half of the pair. In trading, a coin pair represents the exchange rate between two cryptocurrencies (or a crypto and a fiat currency).

For example:

ETH/BTC: This pair shows how much Bitcoin you need to buy one Ethereum.

BTC/USDT: This shows how much Tether (a stablecoin pegged to the US dollar) you need to buy one Bitcoin.

So, when someone talks about the BTC coin pair, they’re usually referring to trades where Bitcoin is either being bought or sold in exchange for another cryptocurrency or fiat-like asset. It's one of the most important base currencies in crypto markets because it’s widely used for pricing and liquidity.