A Pair in trading currencies, regardless if it is digital or actual currency, pertains to the trade of one currency to another. If we are talking about national currencies, then we are referring to Foreign Exchange. This entails that the currencies be exchanged through a position basis. What this means is that all trading done in the context of the Forex market, may it be selling, buying, or outright trading will be administered through currency pairs.
The Pair is defined the same within the context of cryptocurrencies. Regardless of nation of origin, digital coins can also be traded in pairs, for instance, BTC/ETH. But unlike Forex, crypto trading is done through specific online exchanges with transactions recorded through electronic ledgers.