The gap between the bid and the ask — the distance between the best available buy price and the best available sell price.
Full Explanation
The spread is the space between the highest price a buyer will currently pay and the lowest price a seller will currently accept. It is not the same thing as a broker commission. It is a real feature of a two-sided market: buyers are bidding below sellers' offers, and a transaction happens when one side accepts the other side's price.
If you buy at market, you cross the spread and transact at the ask. If you sell at market, you transact at the bid. This is why a position can begin slightly negative even when price has not visibly moved. The market has two prices while your chart often shows only one.
The spread can widen or narrow as available liquidity changes. That makes it especially relevant around tight stops, precise entries, thin trading periods, and news events. Treat it as part of the market's structure, not as an invisible fee pasted onto the chart.