A positive attempt by the defenders to defeat a contract by taking or establishing tricks, usually involving the risk of losing tricks. This is the opposite to a passive defence where the emphasis is on not giving tricks away but waiting for the declarer to lose tricks.
South plays in 4♥ and West leads the ♣J. Winning with the ace, East sees the danger that declarer intends to throw losers on dummy’s diamonds and so makes the active defence of switching to the ♠3. If West holds the ♠A, the defenders will be able to cash some spade tricks.