Definitions for "Best bid"
The highest price any buyer has declared that they are willing to pay at a given time for a given security. see also bid, inside spread.
The highest bid price on an order book for a specified share.
The highest bid price for a specific tradable instrument (i.e. the highest price any buyer has declared that they are willing to pay for a specific security at a given time). Determined by the current quotes and orders in the trading system.