Spread
The gap between the highest price buyers are bidding and the lowest price sellers are asking on an exchange.
The spread is the distance between the best bid (the most any buyer is currently offering) and the best ask (the least any seller will currently take). If the top of the book shows buyers at £98 and sellers at £100, the spread is £2.
Every live market has one, and its width is a quick health check. Heavily traded pairs like bitcoin against the dollar run spreads of pennies. A token far down the rankings can carry a spread of several per cent, which means buying and immediately selling would lose you that much before fees.
The spread is also who pays whom. A market order crosses the spread and pays it; a resting limit order sits inside the book and, when filled, effectively collects it. Exchange fee schedules often mirror this by charging takers more than makers.
Quoted-price purchases fold everything into one number instead: the rate shown includes the provider's margin, so there is no separate spread to cross mid-purchase.