Spread (bid-ask)
The gap between the best bid to buy and the best ask to sell in a market.
The bid is the highest price a buyer will pay; the ask is the lowest price a seller will accept. The spread is the difference between them. A narrow spread means many participants are willing to trade near the current price; a wide one means liquidity is thin or uncertainty is high.
On our site, the spread is shown on quote pages, often in both currency and basis points. Compare it with the asset's own usual spread, not with another asset's.
Traders care because the spread is a hidden cost. Buying at the ask and selling at the bid loses the spread on every round trip, which matters most for short holding periods and large orders.
Worked example (illustrative numbers, not real data): bid 99.95, ask 100.05. The spread is 0.10, or 0.10 percent of the 100 midpoint. A round trip that buys at the ask and sells at the bid costs about 0.10 per share before fees.