Quote currency
The second asset in a pair — the one the price is expressed in.
In plain English
Profit and loss on a trade arrive in the quote currency. If the quote is not the currency you think in, your real result also depends on that exchange rate.
Example
On ETH/USDT, USDT is the quote currency; a move from 3,000 to 3,100 is a 100 USDT move per ETH.