Decaffeinated describes coffee or tea that has had its caffeine removed. Caffeine is the natural substance that can keep you awake and make your heart jump. Shops sell decaffeinated coffee for people who like the taste but not the buzz — people who sleep badly, people who already had three cups, people who have been told to cut down. Though actually, the process does not usually take every last bit of caffeine out, even when a label talks as if it is 'without caffeine'. More careful and more formal than decaf; you will see this spelling on packets.