It stands for national best bid and offer; a quote that reports best terms available for trades at the national level- i.e., the highest bid price and lowest ask (offered) price in a security. It is a system that requires brokers and brokerages to execute customer orders at the best available ask price (offer price) for buy orders, and the best available bid price for sell orders.

This price quotation system is maintained by an exchange regulation (the United States Securities and Exchange Commission, where the best bid (BB) and best offer (BO) prices are calculated and disseminated on a current and continuing basis by a plan processor pursuant to an effective national market system.

As a quotation system, NBBO is designed to guarantee the best available ask or bid price when brokers buy or sell securities for customers. It reports best prices available for each security at any given time aggregated across all market centers nation-wide.