// GLOSSARY

Order book

The live list of resting buy (bid) and sell (ask) orders for a market at each price level. Book depth determines how much size can trade before the price moves — the central constraint in thin prediction markets.

The order book shows every resting order by price level: bids on one side, asks on the other, with the gap between the best of each being the spread.

Worked example

A Kalshi book might show 500 contracts offered at 43¢, 800 at 44¢, and 1,200 at 45¢. A market buy of 2,000 contracts sweeps all three levels — your last fills are 2¢ worse than your first. That price impact is slippage, and it's why size in thin books should be worked, not swept.

Both Kalshi and Polymarket expose the live book over REST and WebSocket — see the API pillar guide.