Spread
The gap between the buy and sell price of an instrument - the broker's built-in cost of trading.
Every instrument quotes two prices: the bid (what you can sell at) and the ask (what you can buy at). The spread is the difference. Buy at the ask and you are instantly down by the spread until the market moves in your favour.
Spreads are quoted in pips for currencies and in points or cents elsewhere. A broker advertising 'from 0.0 pips' is quoting its best-case raw spread on the most liquid pair at the most liquid time of day - typically paired with a separate commission. A 'zero commission' account usually has the cost folded into a wider spread instead.
Spreads widen around news releases, at market open and overnight when liquidity thins out. If you trade frequently, the spread is likely your single biggest recurring cost, so compare the all-in figure - spread plus commission - rather than the headline number.
Related terms
The smallest standard price increment in a currency pair - usually the fourth decimal place.
A bank or institution that continuously quotes buy and sell prices your broker can route orders to.
Two broker models: an ECN routes your order to outside liquidity, a market maker takes the other side itself.