Slippage
The difference between the price you expected and the price you actually got — worse on wide, illiquid option spreads.
Slippage is the gap between the price you expected to pay (or receive) and the price your order actually filled at. It shows up most on market orders and in fast-moving or thinly traded options, where the quote you saw a split second ago is no longer available by the time your order reaches the exchange.
Say a call is quoted 2.00 bid, 2.20 ask, and you send a market order to buy. You might reasonably expect the mid at 2.10, but the ask ticks up and you fill at 2.25. That extra 0.15 per share, 15 dollars on one contract, is slippage. On a single trade it feels minor, but for anyone trading frequently it quietly eats into results.
The common mistake is using market orders on options with wide spreads or low volume, especially far out-of-the-money strikes and weeklies near expiration. A limit order at or near the mid caps what you pay and lets you walk away when the fill is bad, which is usually the better default.
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Educational use only. Quotes are delayed ~15 minutes and nothing here is financial advice. Options trading involves substantial risk of loss.