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The difference between the bid and the ask prices.
For assets where there is a lot of liquidity, the bid-ask spread will be tight. However, when securities have low liquidity, this can get rather wide. The best example of this is real estate where a property could see a big separation between buyer and seller.
High volume assets, such as stocks, will often see the little difference between market and limit orders. This is not the case with low liquidity.