The highest price at which market makers, in a given market, offer to buy an investment (e.g., a security: a share of stock, a bond, etc.) from a market participant at a specific time or during a particular time window. It is the highest bid price in the bid queue: on arrival of a bid price that is equal or better to the currently highest buy offer, the exchange would match the bids at the best bid, on a first-come, first filled basis.

For example, a sell order is expected to be executed at the best bid, which is by nature not constant: it always changes in reaction to movement in the supply and demand for a specific security.

The best bid is one leg in the bid-ask spread (the other is the best ask).