In finance, what is the gap between the highest bid and lowest ask price called?

In finance, the gap between the highest bid and lowest ask price is called the bid-ask spread.

The bid is the highest price a buyer currently offers, while the ask is the lowest price at which a seller is willing to sell. The spread is the ask minus the bid. For example, if a stock's best bid is $49.98 and its best ask is $50.02, the bid-ask spread is four cents.

The spread is a practical measure of market liquidity and an immediate trading cost. Highly traded assets usually have narrow spreads because many buyers and sellers compete; less liquid or more volatile assets often have wider ones. A quote may also be described as a bid-offer spread. It is not the same as a margin, which can mean borrowed funds or profit, nor a premium or discount, which describe relative pricing in other contexts.