A pair in crypto trading is covered in detail under cryptocurrency pair, which explains how exchanges list and price them. This page explains the short word as traders use it across all markets.
A pair is any two assets whose prices are quoted against each other, written with a slash, such as BTC/USDT or EUR/USD. The first asset is the base and the second is the quote: the price shows how many units of the quote asset are needed to buy one unit of the base. If BTC/USDT shows 60,000, one bitcoin costs 60,000 units of the stablecoin USDT. Buying the pair means buying the base and paying with the quote; selling means the opposite.
The same convention comes from the foreign exchange market, where every trade is a currency pair. On crypto exchanges, pairs against stablecoins and bitcoin are the most liquid, while a token with few pairs may need to be traded in two steps. On decentralised exchanges, each pair usually corresponds to a liquidity pool holding both assets.
"Pair" also appears in "pairs trading", a separate market-neutral strategy that buys one asset and sells a related one, betting that the gap between them will narrow. That idea is a form of statistical arbitrage rather than a description of how a price is quoted. Activity in a given pair is measured by its trading volume.