The lowest price at which market makers, in a given market, offer to sell an investment (e.g., a security: a share of stock, a bond, etc.) to a market participant at a specific time or during a particular time window. It is the lowest offer in the ask queue: as soon as a bidder offers a price that is equal or better to the currently lowest sell offer, the exchange would match the offers at the best ask, on a first-come, first filled basis.
For example, a buy order is expected to be executed at the best ask, which is by nature not constant: it always changes in reaction to movement in the supply and demand for a specific security.
The best ask is one leg in the bid-ask spread (the other is the best bid).
