Price Impact is the change in price caused by your trade. Price Slippage is the change in price caused by other traders. When you submit your order you need to set your Slippage Tolerance higher than 0% so that if someone else pays higher gas fee than you and their trade is executed before yours, your transaction doesn't fail and you don't waste gas.

An increase in price causes a decrease in demand, while a decrease in price causes an increase in demand. Deflation is a steady decline in the overall price level of goods and services in the economy over a certain period of time.