A term applying to the SEC requirement that brokers must guarantee customers the best available ask price when they buy securities and the best available bid price when they sell securities. |||NBBO is the bid and ask the average person will see. Day traders usually use Level 2 market maker screens to see ALL the bids and offers for a particular stock. The NBBO is updated throughout the day to show the highest and lowest offers for a security in all exchanges and market makers.