The second currency in a pair (e.g. USD in EUR/USD), showing how much of it is needed to buy one unit of the base currency.
The quote currency is also called the counter currency. If EUR/USD is 1.0850, you need 1.0850 US dollars to buy one euro. Profit and loss on EUR/USD positions are realised in USD.
For pairs where USD is the quote currency (most majors), pip value is fixed at USD 10 per standard lot, making position-sizing arithmetic straightforward.