Bid-ask spread
The gap between the highest price a buyer will pay and the lowest price a seller will take for a stock or ETF.
You pay a little of the spread every time you buy or sell, so it works like a small hidden trading cost.
Large, busy funds usually have very small spreads. Thinly traded funds can have wider ones, especially in a fast market. Trading rarely keeps the cost tiny.
Example
If an ETF is quoted at $79.98 to buy from you and $80.02 to sell to you, the spread is $80.02 − $79.98 = $0.04 a share.