National Best Bid and Offer - NBBO

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.

Investopedia Says:
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.

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