Bid-ask spread

The bid-ask spread is the difference between the best (highest) bid and the best (lowest) ask for an asset at a given moment. It represents the gap between what buyers are willing to pay and what sellers are willing to accept. A narrow spread generally reflects an actively traded, liquid market, while a wide spread often signals thinner trading or greater uncertainty.

The spread is effectively a cost of transacting immediately, because a buyer who takes the best ask and a seller who takes the best bid trade at slightly different prices. It is a mechanical property of the order book, not a fee charged by the venue.