Slippage
In short Slippage is the gap between the price quoted for a trade and the price it actually executes at, caused by price moves between quote and execution.
Slippage is the difference between the price you were quoted when you set up a swap and the price your trade actually executed at. It can work against you or, less often, in your favor, and it can be zero if nothing changes between the quote and execution. It tends to be larger when a token is thinly traded or its price is moving fast.
Keep it separate from price impact. On a DEX, the quote you see usually already includes price impact, meaning how far your own order moves the pool's price. In a basic constant-product pool that follows from the ratio of the two tokens, though other designs, like concentrated liquidity, price differently. Slippage is what happens after the quote: other trades land first, the pool's state shifts, and your fill comes in worse. Slippage tolerance caps how much worse than the quote you will accept, and if the result falls outside it, the transaction reverts and you lose only the network fee.
Tolerance is a tradeoff. Set too low on a volatile token, swaps keep failing and cost you fees each time. Set too high, it gives bots room for a sandwich attack, where one buys just before your trade and sells just after. Profit from that depends on pool mechanics, trade sizes, fees, and competition between bots, so a high tolerance is not a fixed loss, but it removes your safety limit. Many small tokens also charge a transfer tax, and that tax is a separate cost from slippage even if you have to raise your tolerance to get a swap through.
Before buying a small token, check how much liquidity sits in the pool, since a shallow pool can move a lot on a modest order, whether you are buying or selling. Read the price impact your interface shows, which is separate from your tolerance setting. Splitting a large order into pieces against the same pool generally does not reduce total price impact and adds fees, unless liquidity refills or the price recovers between pieces. A limit order, where available, or routing across several pools can help more. Reset your tolerance after any trade where you raised it.