Price Impact
Plain English
How much your trade affects the price. When you buy a lot of something, the price goes up. When you sell a lot, it goes down. Bigger trades = bigger impact. Like buying all the apples at a market - the seller raises the price because there's fewer left.
Technical
The change in asset price caused by executing a trade, particularly significant in AMM-based DEXs due to constant product formula (x*y=k). Larger trades relative to liquidity pool size cause exponential price impact. Calculated before execution to estimate actual execution price versus quoted price.