Glossary
Slippage
The gap between the price you expected and the average you actually got. On Polymarket it is almost always thin depth at the touch.
Slippage is the gap between the price you thought you would get and the average price you actually got. On Polymarket it is almost always a book problem: not enough size at the touch, so your take walks to worse prices. A 1¢ spread on a liquid 15-minute crypto market is noise. A 6¢ hole under a Supreme Court name is the real cost of getting out.
Displayed depth can vanish as you hit it — other makers cancel — so the slippage you model from a screenshot is a lower bound. You also see apparent slippage in analytics when a position is marked to last trade while the bid you could actually hit is two ticks worse. That is why Polydata’s methodology treats headline PnL as realized cash plus resolution, not as a live liquidation price.
To read slippage after the fact, compare a whale print to the prior mid and to the size. Repeated takes in one direction are how thin books become movers. Tight spread plus thick size is the opposite: you can be wrong on the thesis and still get a fair fill. Research only — we do not route your size.