What term names the difference between a currency dealer's buy price and sell price?

The difference between a currency dealer’s buy price and sell price is called the bid-ask spread.

The bid is the price at which a dealer or market maker will buy the base currency, while the ask—also called the offer—is the price at which the dealer will sell it. The ask is normally higher than the bid, and the difference between them is the spread. In EUR/USD, a quote of 1.5760/1.5763 therefore has a three-pip spread.

The spread is a trading cost for someone seeking immediate execution. A trader selling generally receives the bid; a trader buying generally pays the ask. A round trip can therefore lose the spread even if the market price has not changed.

Spreads often become narrower in highly liquid markets and wider during volatile periods, thin trading, or major economic announcements. They are commonly confused with commission, but a broker can charge a separate commission in addition to, or instead of, marking up the spread.