A bad bishop is one whose mobility is restricted by its own pawns.

A bad bishop is one whose mobility is restricted by its own pawns. Sometimes such a bishop's mobility is so limited that it resembles little more than a "big pawn." Since that's almost like being down a piece, a player will strive to improve the bad bishop or trade it away. Conversely, players also attempt to trade mediocre pieces for an opponent's "good" bishop.