The bid-ask spread, or “spread”, is the gap between the bid and the ask (see section of Bid and Ask). The bid is the best price a seller can receive if they wish to sell immediately, and an ask is the best price a buyer can receive if they wish to buy immediately. Suppose someone places a limit order to purchase Security X for $44.52/share and that is the highest price on the order book. $44.52 is thus the bid, since if a seller wishes to sell, the most they could get is $44.52 a share. However, the lowest resting order to sell Security X is $44.55/share, so if someone wanted to buy Security X immediately they’d have to pay $44.55/share ($44.55 is thus the ask). Thus the spread is 3 cents. Highly liquid markets tend to have smaller, or “tighter” spreads.