To discard is to fail to follow to the suit that declarer has led and thereby playing a card from another suit.....but not a trump. Good defensive signaling makes for a good partnership. Don’t forget that every card played by the defenders has a purpose. It can take a trick, it can set up a trick, or it can communicate information to partner. Your partner will appreciate it if you can guide her to the proper defense in particular if you possess info that she does not.  Finding and sending the appropriate signal to partner can often be the difference between a top and a bottom on a particular board.