A style of defence that attempts to avoid establishing tricks for declarer as opposed to actively trying to establish defensive tricks.
This style is most likely to be right when dummy is flat, lacking a source of tricks. To defend passively you would usually play on suits where your holding is worthless or where the previous play of the suit has already determined how many tricks each side can make in the suit.
South plays in 4♠. West leads the five of clubs and the ace wins. Returning a club to dummy’s now bare king is passive and gives nothing away. Opening up either red suit would be highly dangerous.
If declarer next plays a spade to the queen, West takes the ace and, again refusing to open up one of the red suits, returns a low spade, thereby continuing the passive defence.