Spread
The difference between the best ask price and the best bid price in an order book, representing the cost of an immediate round-trip trade.
Definition
The spread is the gap between the lowest resting ask (the price a taker pays to buy) and the highest resting bid (the price a taker receives to sell). A tight spread indicates a liquid market; a wide spread indicates low liquidity or high uncertainty. The spread is the minimum round-trip cost paid by a taker who enters and immediately exits.
In practice
On a liquid Polymarket binary, the YES token might show a best bid of $0.49 and a best ask of $0.51 — a 2-cent spread. A taker who buys at $0.51 and sells at $0.49 loses 2 cents on the round trip before fees. Maker strategies profit by sitting on both sides of the spread and collecting the difference when filled. Signal-based takers must have an expected price move larger than the spread plus fees to be profitable. Predtools bots check the current spread before entering and skip trades where the spread alone would eliminate the edge.