a set of agreements about the meanings of the different bids that the players can make during the auction phase of each hand

A set of defined meanings for bids and sequences of bids, e.g. Acol, Precision Club, Standard American.

A Bidding system is a collection of agreements and conventions describing the meanings of bids used in contract bridge. The purpose of bids is to both describe a bridge hand and propose a contract (Any bid becomes a contract if followed by three successive passes, therefore every bridge bid is a potential contract).